There are three people in the race to replace John Key - his right-hand man Bill English, Health Minister Jonathan Coleman and Judith 'Crusher' Collins.
MPs have emerged from their normal caucus meeting, where they made their positions clear on the upcoming vacancy.
Bill English: Nathan Guy, Michael Woodhouse, Nuk Korako, Louise Upston, Hekia Parata, John Key
No comment: Amy Adams, Chester Borrows, Chris Bishop, Scott Simpson, Alfred Ngaro, Mark Mitchell, David Bennett, Murray McCully, Anne Tolley
Transport Minister Simon Bridges confirms he won't be running for leader.
Education Minister Hekia Parata backs Mr English.
Paula Bennett still isn't ruling out a run for the leadership, which could mean there would be four contenders in the increasingly heated competition.
Amy Adams says she has no plans to run for the leadership.
"I haven't made any decisions. It's still very early in the process, but I'm not intending to stand as leader."
She says any suggestions of her being on the ticket with declared contenders is "speculation".
Judith 'Crusher' Collins will be taking on Mr English and Mr Coleman for the leadership.
She says the party needs someone who can "connect to New Zealanders, whether they're men, women or of any ethnicity".
"I believe I can do that.
"I think it's important that the best people for the jobs get them and I happen to think I'm one of those people."
Ms Collins says she wouldn't put her name forward unless she thought she had a "very good chance" at taking the party to victory in the 2017 election.
Auckland Central MP Nikki Kaye has been welcomed back to Parliament having left earlier this year to seek treatment for breast cancer. She confirmed she will stand in her seat again next year, and backed Mr English for Prime Minister.
After speculation and the backing of the Prime Minister, Finance Minister and Deputy Prime Minister Bill English has confirmed he'll also contest for the job.
Talking to media after the meeting, he said the country has a "huge opportunity" ahead of it.
"We've got choices over the next five to 10 years most countries don't have. I've been intimately involved in the policies of a John Key-led government. I can see fantastic opportunities for a stronger economic performance, of spreading the benefits of growth for all New Zealanders."
Following the caucus meeting, Mr Coleman says there is a mood for change and has confirmed his bid for the leadership.
"It really is time to build on the gains of the last few years and the question before caucus is 'how do we best do that?'
"I feel it needs generational change, it's going to need new thinking in policy areas, it's going to need new personnel - so combining the best of the current line-up and those who are coming through the caucus."
National are in disarray, with six senior MPs refusing to rule out challenging for the leadership left vacant by John Key.
It seems certain there will now be a challenge to Bill English - if he even puts his hand up at all.
Not ruling out leadership challenge:
Maggie Barry, who only arrived back in New Zealand after travelling to Mexico, said she was still in shock over the decision and the new leader would be a matter for caucus.
Bill English has walked right on past a waiting press pack on his way to the party's important caucus meeting about the impending leadership change.
Transport Minister Simon Bridges hasn't ruled out a bid either.
"I think I'm going to keep my powder dry and what's important to me is that we go in and decide in there.
"These issues are far too important to be dealt with in the media."
The competition is truly on now with Health Minister Jonathan Coleman saying he'd been ringing around to gauge support for a possible leadership bid.
Bay of Plenty MP Todd Muller joined the chorus for change, saying there was "probably an appetite for a contest".
"We're reflecting on the national party direction for the next three years and possibly beyond."
Steven Joyce seems to have ruled out a bid for the leadership and deputy leadership.
He says he has "very little aspiration for these sorts of roles" and would rather support whoever's in charge and serve the public
Mr Key didn't think he'd play a major role in next year's election.
Mr Key responded to the National MP who said there was a mood for change, saying "if people want to put their name forward, they'll put their name forward".
"I've put my cards on the table when it comes to the leadership, but in the end it is up to others."
Mr Key says he'd probably leave a bottle of Champagne on the new leader's desk and note wishing them well.
Mr Key says he'll chair Tuesday's caucus meetings as normal until it turns to discussion about the leadership.
"There will be decisions they'll need to make about the voting process, and all those sorts of things - what the rules are. I'm going to leave at the point at which we do that. It seems appropriate for me to do that."
He said he hadn't spoken to Mr English about a possible leadership bid since his shock resignation.
When asked whether his endorsement for Mr English would cause some resentment among others, Mr Key said he spoke to each member of Cabinet about his decision and that he'd publicly back the Deputy Prime Minister for the job.
"What I was doing was trying to avoid being chased around the paddock for the next six days and I just thought it was best for me to be frank and straight."
A National MP told Patrick Gower there's a "huge mood for change" in the party, and that there should be a competition.
Newshub political editor Patrick Gower was the first to speak to Bill English about his leadership bid on Tuesday. Mr English coyly said: "You'll have to wait and see."
Asked repeatedly whether he would run for leader of the National Party and Prime Minister, Mr English would only repeat his line.
"Wait and see" is his standby line in the guessing game ahead of the eight Budgets he has delivered.
He's spoken to his family about the possible promotion already.
"We had a good chat about it, but I'll need to talk to the caucus first."
MP Jono Naylor says the Bill English of 2002 (when he was last the National Party leader) and 2016 are the same person but he's learned a lot since then. "I wouldn't want people to judge me on what I was like 15 years ago."
The National Party president is calling Mr Key's resignation an "exciting time" for the party.
But Peter Goodfellow says if other parties are happy at the loss of "brand Key", they're looking in the wrong place.
"National is the strongest political party by far in New Zealand, or even in countries that we compare ourselves to. We have close to 30,000 members and many, many activists amongst that, and many people with good policy ideas."
MPs Alfred Ngaro, Mark Mitchell, David Bennett arrived together in the same car and were all tight-lipped about who they're backing.
"For us, we're new MPs, in a sense of being here for five years. We've never experienced this before," Mr Ngaro says.
She's been touted as a leadership contender, but Paula Bennett hasn't declared her bid publicly. The name of her Facebook page has changed this morning from being focused on her electorate to something more general.
Former National Party leader Don Brash has backed Judith Collins as the new Prime Minister, saying she's a "person of integrity" who will "deal with tough issues".
Two senior ministers, Nathan Guy and Michael Woodhouse told Newshub they backed Bill English as Prime Minister, but the former says that's an issue for caucus.
Chester Borrows says he's shocked by the resignation and isn't willing to let on who he'd like to see step up to the plate, and Chris Bishop says he'll only be talking to his colleagues about who is preferences are.
Māori Party co-leader Te Ururoa Flavell says he was shocked and "a little bit disappointed", but an "understandable" decision. He didn't back a particular person for Prime Minister, saying the party could work with "pretty much most of them".
List MP Nuk Korako said there was "a lot of mana in what the Prime Minister did. I think he's giving us a real opportunity to do the transition seamlessly and then we can all work together again".
He backed Bill English, saying he's been "good for Māori", particularly in the Treaty settlement process.
Speaking to Paul Henry on Tuesday morning, Joseph Parker - a champion boxer and nephew to National MP Judith Collins - said the Beehive is not a ring he'd want to step into.
"Maybe I'd need to go back and study if I want to be a politician, that's not my area."
He sent a text message to Ms Collins while on air.
"Hi aunty, Joseph here, I know all about fights, your fight begins at 10 o'clock this morning."
One of the touted contenders for Mr Key's job, Amy Adams, was refusing to comment on her own ambitions when asked by Newshub on her way into Parliament on Tuesday morning. She said it was a decision for her caucus to make.
Mr Key's main rival, Labour leader Andrew Little says while his shock departure was "dramatic", he's adamant it won't change what's wrong with New Zealand.
"I think about the people who are missing out - homelessness is still there, people can't get the hospital treatment. None of that has changed…what we will be campaigning on next year hasn't changed."
Mr Little says the weekend's Mt Roskill by-election, in which their candidate Michael Wood annihilated National's Parmjeet Parmar show Mr Key's star was waning.
"The Prime Minister spent seven days campaigning in that by-election, it was all about him pulling out the National Party vote given that that seat - there are more National voters than Labour voters in that seat - and the truth is that by Saturday night, he hadn't pulled out those voters.
"It was a slam dunk for us, but the Prime Minister's star power simply had not worked. I picked that up for a long time - the truth is the shine's gone off him."
Political commentators, left-leaning Chris Trotter and right-leaning Michelle Boag, are left stunned by Mr Key's decision.
"What is absolutely stunning, and I think that is the word to use, about John Key's departure is that he has gone at a time of his own choosing and undoubtedly at the top of his game because you've got to remind yourself constantly that this is a man who has been in power for eight years, whose party is nudging 50 percent in the public opinion polls, whose country's economy is the envy of the OECD and who was set fair for a fourth term in Government. People in that position don't usually quit," Mr Trotter told Paul Henry.
Ms Boag says while there'll be "some change", Mr Key's departure won't mean an easy road for Labour.
"What we have to remember is New Zealand politics is all about coalitions. It's all very well the Labour Party saying 'oh great John Key's gone' but they're still only on 23 percent they haven't been on over 30 percent for a long time."
She's picking Bill English:
"Bill has been the policy architect of this Government, he's been the chief operating officer. I think Bill can do it… He is my choice, I do hope that he is the next leader. I think she [Paula Bennett] has a very good chance of being deputy."
Mr Key's incessant critic paid tribute to the outgoing Prime Minister, calling him the "Teflon man".
"No matter what went wrong, nothing seemed to stick with him and that's a very lucky characteristic and a very likable one if you're a National Party voter.
"You've got to give credit where credit is due - it's not a time to analyse what it all means and I wish him and his family all the best in his retirement. But he had some extraordinary good luck, including for the most part an incompetent Opposition."
But with New Zealand's most preferred Prime Minister on the way out, other parties would be "seriously deluded" if they think it's good news.
"You've got to work at it, you've got to have plans and you have to have strategies and the capacity to get out there and hear people."
New Zealand First leader Winston Peters also thinks Mr English will be the new Prime Minister, but privately could think differently.
"[Mr English] still has the credibility and the mana within his party and if [caucus] snub him next Monday, it'll be astonishing."
He didn't want to tell Paul Henry who his personal choice for the top job would be.
"I'm happy to write it down and you open it net Tuesday and I'll tell you who the person should have been or should be," he joked.
He was "certain" Tuesday's closed-door caucus meeting would be a bloodbath.
"The ambition of some people in politics know no bounds, as you well know. It's not based on talent, commitment and ability as you also know."
John Key told Paul Henry on Tuesday that a change at the top could be a "healthy thing".
"By me leaving the opportunities for the rest of the caucus are enormous."
While Mr Key is banking on Mr English to replace him, the ultimate decision isn't his to make.
"He thinks a lot like I do, he's a thoroughly decent human being, he'd be a great Prime Minister. But in the end if it's someone else in the caucus - and there are many other potentials that'd be great - they'll have my 110 percent support."
John Key is expected to lay out how he wants the succession to be handled following his shock resignation on Monday, and potential challengers will make up their minds when they've heard him out.
Mr Key is backing his deputy Bill English as successor – that makes Mr English the hot favourite for the job, although the Deputy Prime Minister and Finance Minister is yet to confirm he's in the running.
The potential challengers include Judith Collins, Paula Bennett and Amy Adams, with Steven Joyce tipped as a possible finance minister.
But Mr Key has a powerful influence in National's 59-member caucus and it's unlikely a vote would go against his chosen successor.
Ms Collins is the only one who has said anything so far, telling Newshub she was "thinking about it" but hadn't yet made her mind up.
Mr Joyce won't comment on any aspect of the leadership, and Ms Bennett is also keeping quiet.
Caucus will vote on its new leader on December 12.
Mr Key stunned parliament and the nation when he announced on Monday he was stepping down after eight years.
"Simply put, it has been for me the most remarkable, satisfying and exciting time of my life," he said.
""But, despite the amazing career I've had in politics, I've never seen myself as a career politician ... there's nothing left in the tank."
He made the call for family reasons as well, saying his wife Bronagh had spent many nights alone and there had been "extraordinary levels of intrusion and pressure" for his children Stephie and Max.
Newshub. / NZN