It's been a tumultuous two years at the helm of the National Party for Simon Bridges.
On Friday, following an emergency caucus meeting in Wellington, Bridges discovered he had been deposed from the job after the Bay of Plenty MP succeeded in the leadership coup he launched this week.
Muller will hope he can get the party back on track ahead of the general election in September, after a recent Newshub-Reid Research poll revealed the Opposition would lose the election to Labour by a landslide as it stands.
There's plenty of work required before then to turn the ship around - but for now let's take a look back at Bridges' time as National leader, and what caused New Zealand's biggest party to crumble so spectacularly under his leadership.
Simon Bridges' term as National leader: A timeline
Simon Bridges became National Party leader after former Prime Minister Bill English stepped down. English had twice led National to defeat in an election, and was stepping aside to give his party a chance to regroup.
Simon Bridges found himself facing poor approval ratings. He made his debut on the preferred Prime Minister ranking at 9 percent, well-behind Jacinda Ardern on 40.2 percent.
A leak to Newshub revealed Simon Bridges spent $113,000 in taxpayer money on limos and hotels in three months during his time travelling the country getting to know voters.
He was then criticised after launching an investigation into the source of leaked expenses, calling for a High Court judge to preside over a full-blown forensic probe, when the leaked data was due to come out anyway.
Simon Bridges was criticised after he described then-National MP Jami-Lee Ross taking leave from Parliament over personal health issues as "embarrassing" - which he later expressed regret over.
Ross came out with a series of allegations against Bridges, lambasting him as unpopular and accusing the National leader of asking him to split up a $100,000 donation from a Chinese businessman into smaller amounts - an allegation Bridges denied.
Ross then released a recording of Bridges calling fellow National MP Maureen Pugh "f***ing useless".
Simon Bridges seemed to be feeling positive about his leadership. When asked by The Project if he would survive 2019 as National leader, he replied: "Absolutely".
A Newshub poll showed support for Simon Bridges had slipped. It revealed National MP Judith Collins polled higher than him as preferred Prime Minister, and that National's overall popularity had dropped.
The Serious Fraud Office confirmed it had received a referral from police about a complaint they received in October 2018 in relation to the disclosure of political donations under the Electoral Act.
National MPs approached Newshub to say Simon Bridges was losing caucus support. MPs were disappointed in his treatment of an "emotional junior staffer", who he blamed for the deletion of a petition against the UN Migration Compact in the wake of the Christchurch terror attack.
Judith Collins then refused to say she was loyal to Simon Bridges after Newshub was told the numbers were firming for her to take a leadership role - a position she had previously put her hand up for.
Simon Bridges had a win for National when he revealed sensitive Budget 2019 documents had been uncovered before the official release date. It turned out National had accessed the information simply by searching for it on the Treasury website.
Simon Bridges was criticised for meeting with Guo Shengkun, known as the head of the Chinese Communist Party (CCP) "secret police". He was interviewed by a state-owned news channel, praising the CCP for taking the country to economic prosperity.
Labour MP and Cabinet minister Willie Jackson said Simon Bridges was improving as National leader, describing him as going "from poor to average".
However Bridges' personal popularity increased in Newshub's poll to reach 10.6 percent against Jacinda Ardern on 38.7 percent, while National overtook Labour in popularity on 43.3 percent compared to Labour on 42.5 percent.
Bridges was then not included in charges laid against four people by the Serious Fraud Office in relation to donation allegations raised by former National MP Jami-Lee Ross.
Some National MPs told Newshub they were unimpressed by the tone Simon Bridges had taken during the COVID-19 crisis, pointing to a highly criticised Facebook post in which he questioned the decision to extend the COVID-19 lockdown.
Newshub then obtained a letter written by senior National MP Nick Smith - with the entire caucus copied in - expressing disappointment in Bridges for setting up a new COVID-19 policy team that clashed with Smith's existing role.
National MPs told Newshub they had not been shown internal polling since February, and at that time Todd Muller was rumoured to be considering a leadership bid.
A devastating Newshub-Reid Research Poll showed National had dropped to 30.6 percent, compared to Labour on 56.5 percent. Bridges' ranking as preferred PM also dropped to 4.5 percent against Jacinda Ardern on 59.5 percent.
National MPs told Newshub the numbers were being done to roll Bridges, with one of them - David Bennett - even putting it in writing, telling a constituent "we're working on it".
Bridges confirmed two National MPs were seeking to challenge his leadership as well as deputy leader Paula Bennett, and a source told Newshub it was Bay of Plenty MP Todd Muller and Auckland Central MP Nikki Kaye respectively.
A source then confirmed to Newshub that National MPs would return to Wellington for an emergency caucus meeting after the leadership of Bridges and Bennett was challenged.
Todd Muller confirmed his formal leadership challenge against Simon Bridges in an email seen by Newshub sent to his National Party colleagues, in which he said Bridges could not win the September election.
MPs' expenses appeared to show Todd Muller had been campaigning for months to roll Simon Bridges, with a sharp spike in his travel and accommodation costs compared to other National MPs.
National's caucus held its emergency meeting on Friday, May 22, at which a vote revealed Muller had more support than Bridges, and he was ousted as leader.