What we know:
- Winds reached upwards of 260km/h in Tonga overnight
- Damage includes roofs ripped from houses, flooding, fallen trees and cut power
- New Zealand is giving an initial $750,000 towards relief efforts
- The cyclone will hit Fiji's remote southern islands on Tuesday afternoon
- The front could move towards New Zealand as an ex-tropical cyclone next week
- Cyclone Gita hurtles towards Fiji with 295km/h winds
- Cyclone Gita: New Zealand could face 'direct hit'
Live updates have now ended.
12:15pm - Cyclone Gita is projected to reach category 5, the highest storm category, by the time it passes Fiji's southern islands.
The storm is currently at category 4.
12:00pm - The Tongan police headquarters in Longolongo was badly hit by Cyclone Gita overnight.
Fallen trees lie scattered outside the flooded building, and executives are still working to assess the extent of the damage.
The station is trying to make contact with the 'Eua police station, in the worst-affected eastern part of Tonga, but has had no luck so far.
The last contact was made at 11:30pm on Monday.
11:50am - The Tongan Red Cross is trying to reach the worst affected areas of the island nation.
The eastern district was badly hit, with locals completely cut off by power lines, trees and flooding.
Volunteers are working their way through the damage.
11:30am - There are concerns for the smaller islands of Fiji as Cyclone Gita fast approaches.
Oxfam's country director for Tonga, Jane Foster, is in Suva, Fiji, and says the southern islands of the small nation could take the hit.
"It's unlikely that the main islands will be impacted greatly by the storm," she told Newshub.
"It's probably just going to be sort of wind and rain."
"But there are a number of small islands really quite far to the south of Fiji and they are expected to get the major impact."
The storm will touch down in Fiji this afternoon.
11:00am - Newshub Pacific Affairs correspondent Michael Morrah is in a small village east of Tonga's capital and says the devastation is "immense".
"Homes have been completely flattened and parliament's building has collapsed," he told RadioLive.
"There's powerlines that have just been snapped in half, and have completely buckled over and homes and businesses around there are underwater."
But Morrah says it hasn't dampened spirits.
"Despite losing so much, talking to people this morning I was just amazed by their resilient attitude, their hardiness and their willingness to get on and rebuild.
"People who have lost homes are simply saying, we'll build a new one.
"That sort of attitude is really quite humbling and surprising given what they've just been through."
10:20am - Jacinda Ardern says a surveillance plane could be deployed within hours to survey the damage over Tonga.
An emergency relief and response team aircraft will take up to 12 hours to load and deploy.
"We're essentially just waiting for an indication from the Tongan government of their needs."
The $750,000 New Zealand will give to Tonga to support aid effort was approved on Monday - but Ms Ardern says that is a starting point.
"That was simply so we could move immediately. There's no hold up on our end for making sure that resources are available.
"Of course we'll keep reviewing the situation on what's needed."
There are around 90 New Zealanders in Tonga, Ms Ardern says.
"No immediate concerns have been raised" for their safety, but officials will visit major hotels on the island today to double check.
9:40am - Tonga's parliament building has been completely flattened.
Photos on Twitter show the red and white building in pieces.
9:35am - Weather updates for Tonga are coming out of Fiji, as the country's MetService had its roof torn off overnight.
9:25am - The Government is taking its first step to assist with the clean up of Cyclone Gita ravaged Tonga.
An initial 750 thousand dollars is being made available to support relief efforts.
Foreign Affairs Minister Winston Peters says it will be used to meet immediate needs, such as emergency shelter, water and sanitation.
"Initial assessments of the damage are still coming in. However, it is clear that Cyclone Gita has caused significant damage.
"We stand ready to provide additional support as the extent of the damage becomes clear.
"We will also be responding to requests for assistance from New Zealanders in Tonga.
"Given the force of this Cyclone all signs point to a lengthy clean-up effort and our thoughts are with the Government and people of Tonga as they come to terms with the scale of this disaster."
9:00am - Fiji is being warned to hunker down as Cyclone Gita approaches.
A hurricane warning is in place for Ono-i-Lau and Vatoa, with winds averaging 195km/h, and gusts of up to 275km/h expected.
A strong wind warning is in place for the rest of Fiji.
"Destructive winds are likely to begin several hours before the cyclone passes overhead or nearby," the the Fiji Meteorological Service says.
The storm is passing over the water and will touch down in Fiji this afternoon.
8:40am - Around 40 percent of buildings in Tongan capital Nuku'alofa have lost roofs, the Emergency Management Office told Sky News.
It is understood the office will put in an official request for New Zealand and Australia's aid this morning.
8:30am - The United Nations Coordination of Humanitarian Affairs Office (UNOCHA) is on standby to provide assistance to Tonga, and is monitoring the situation.
8:25am - National leader Bill English has tweeted his sympathies to the people of Tonga.
"Our thoughts are with the people of Tonga this morning, who are being confronted by devastating damage caused by Cyclone Gita.
"As always, New Zealand will be here to help. Kia kaha."
8:20am - The Red Cross is preparing to send 300 tarpaulins to Tonga.
8:15am - Newshub Pacific Affairs correspondent Michael Morrah has taken a video of the widespread damage and flooding.
8:00am - Tearfund's chief executive is urging Kiwis not to send containers full of goods to Tonga.
"We simply don't need containers full of junk," Ian McInnes told The AM Show, adding the better way to help is by giving money to aid agencies.
Mr McInnes says he expects a "full-scale response" from the New Zealand Government.
"It's anyone's guess how many homes are down now, livelihoods will have been lost.
"People rely on crops, small livestock, so that's a food source gone."
7:45am - Newshub's pacific correspondent Michael Morrah is on the ground in Tonga's capital.
"It's really serious, it's a major disaster," he told The AM Show from his flooded hotel room.
"Looking out my window this morning I can see homes that have collapsed, roofing iron ripped off, power cables everywhere.
"I can see people sitting out on their roofs looking across the water. It's a mess.
"The power of the cyclone was just incredible. It was most intense around midnight and it seems like the cyclone just sat over the capital for hours. It was frightening to be honest.
"It sounded like a plane's jet engine right outside."
7:35am - Tonga's Emergency Management office says at least one person may be dead and many injured.
A spokesperson told Newshub they have reports of one fatality, but communication is so bad with the islands it cannot yet be confirmed.
7:30am - A church is one of the buildings that has been destroyed.
Photos on Twitter show pews scattered around the wooden building and windows smashed.
7:10am - Video is coming in live on Facebook showing the destruction in a Tongan hospital.
Vaiola Hospital is flooded and appears to be evacuated, apart from the man behind the camera.
A view from a balcony shows trees uprooted and debris scattered on the ground.
In the carpark, vehicles appear to have been thrown by the strong winds that hit overnight.
Hundreds of people are currently watching the livestream and comments are pouring in.
7:00am - A curfew on the CBD of Tongan capital Nuku'alofa has lifted. Locals can now go outside and see the damage.
6:45am - Jacinda Ardern says Defence Force planes and emergency response teams have been on standby all night.
"I've just made contact with our foreign affairs team," she told The AM Show.
"We are at the moment waiting for first light so there's the ability to survey the damage that's been done.
"We have the Defence Force planes at the ready, and they have for instance the ability to bring over equipment if that's what's required, emergency accommodation tarpaulins and so on.
"We'll be waiting for the Tongan government to tell us exactly what their needs are as soon as they're able."
6:40am - The chair of the NZ Tongan Advisory Council has been up all night worrying for his friends and family back home.
"I just try to imagine it," an emotional Melino Maka told The AM Show.
"There's a lot of people there in poor housing.
"By the time daylight [hits] we will see the size of the devastation
"I'm too scared to see what it's like."
"For the Tongan community here I think it's going to be the biggest relief effort we've had to encounter."
Mr Maka says a lot of Tongans may have taken a laid-back attitude to warnings earlier this week.
"I think this cyclone will give them a real wake up for the future… more of this will come."
6:30am - The first photos are emerging of a trail of destruction left behind by Cyclone Gita.
Dawn is breaking over Tonga, revealing what appears to be extensive flooding and fallen trees.
6:00am - New Zealand Red Cross is waiting to see the damage left by Cyclone Gita when the sun rises in Tonga at around 6:30am (NZ time).
Prime Minister Jacinda Ardern says the country has not formally been asked for assistance but is on stand-by.
5:33am - Video shows torrential rain outside Nuku'alofa Hospital.
5:16am - How the cyclone looked from above.
5:05am - Gita is the strongest cyclone to pass so close to the main islands of Tonga in at least 60 years, according to the UK Met Office.
4:58am - The numbers are in, showing just how big the storm was:
- Average wind speeds of 114km/h with gusts of 176km/h
- Around 158mm of rain fell over the past 12 hours
- Swells of up to 4.6m
4:21am - Heartbreaking stories of loss are emerging.
3:45am - Robert Blake livestreamed as the storm reached its height.
3am - Newshub Pacific correspondent Michael Morrah said there's real concern about water contamination because of all the debris flying around. Officials are advising people to boil water.
2:35am - Good news: Latest reports from Tonga say the storm is easing.
2:15am - Reports of damage are emerging on social media.
1:40am - Newshub Pacific correspondent Michael Morrah is on the ground in Tonga and said the wind and rain is really picking up.
"It's quite ferocious actually what we're seeing. Trees and powerlines have already been toppled... it's quite scary, I haven't seen anything like this before."
1:19am - Tongans are livestreaming on Facebook to show the full force of the storm. This video shows ferocious winds with the sound of glass smashing in the background.
12:20am - Members of the public are advised to keep informed about weather developments through radio bulletins and social media.
11:40pm - Tonga's MetService building in Fua'amotu has been damaged in the storm. Weather updates will now come from Fiji.
11:25pm - Officials told Newshub that Tongan residents need to have at least three days' worth of food and water in their homes, and to be prepared to be without power or communications for some time.
10:55pm - Homes in Nuku-alofa are being flooded as the eye of Cyclone Gita passes over the capital.
10:30pm - Winds are intensifying, with reports of power lines being knocked down by falling trees.
The eye of the storm is expected to arrive over Nukuʻalofa, Tonga's capital, at around midnight.
10:10pm - Cyclone Gita is expected to make landfall on Tongatapu soon. This occurs when the eye of the storm moves over land and is when most damage tends to be inflicted.
9:35pm - NIWA says because there won't be an official landfall, Tonga will remain in Cyclone Gita's dangerous "eye wall" for longer.
9:10pm - Tonga's main island of Tongatapu is without power.
On Monday Tongan Police Commissioner Stephen Caldwell said the tropical storm "could be the most powerful in the country's history."
Acting Prime Minister Semisi Sika declared an official state of emergency on Monday morning, and the storm is expected to hit the island in the next few hours.
The tropical cyclone will bring winds of more than 200km/h and is expected to upgrade to a category 5 storm overnight.
Cyclone Gita passed through Samoa on Sunday and Monday, causing widespread flooding and evacuations.
Staff at the New Zealand High Commission in Nuku'alofa have been instructed to stay indoors.
The High Commission in Tonga's capital has been closed as Cyclone Gita converges on the island nation, and it will remain closed tomorrow too.
MFAT is advising Kiwis in Tonga to follow the advice of local authorities and watch weather reports, while visitors in accomodation should follow the guidance of management.
Prime Minister Jacinda Ardern said the New Zealand government is ready to step in and help Pacific Islands affected by Cyclone Gita.
"My latest advice is that we haven't been asked for direct assistance from Samoa as a result of the damage they've received," Prime Minister Jacinda Ardern said on Monday.
"But we are on standby, ready and waiting to discuss with them any needs they might have. The same of course goes for Tonga - we're waiting to see the impact there and staying in very close contact."