Readmore: Cyclone Gita - what does it mean for NZ's weather?
Cyclone described as the strongest to hit Tonga in its history.
Some estimates that 75% of homes in capital Nukuʻalofa destroyed.
Fresh water and power supplies are out in many areas.
New Zealand Defence Force planes and emergency response teams are on standby
If you're a Kiwi in Tonga and require consular assistance contact the NZ High Commission on +676 23122, +676 881 7022 (after hours) or nzhctonga@mfat.govt.nz
Tonga's residents are waking to the carnage left by Cyclone Gita in what was feared to be the fiercest storm in the tiny island nation's recorded history.
Metservice New Zealand lead meteorologist Michael Martens said the eye of the cyclone passed the main Tongan island Tongatapu overnight with winds of up to 195kmh. At 5.30am today it was about 140km to the southwest.
"The eye of the storm passed just below Tonga, which can be relatively calm, but they were hit by the eye wall, which garners most of the wind strength and rainfall."
There are now fears for Fiji, as the cyclone tracks its way. Gita is expected to hit the southern Lau and Kadavu islands, with average winds of up to 215kmh expected by midday.
Most weather forecasting models predict the giant storm will track towards New Zealand and move across central parts of the country early next week.
Damage to Tonga is yet to be assessed, but New Zealander Joanna Bourke, who lives there, this morning said she is "fearing the worst".
She told Newstalk ZB Early Edition host Kate Hawkesby the night was "horrible" as the cyclone "roared".
Tonga has been hit really badly, she says, although she will have to assess damage to her own home after dawn.
She had just looked outside with the use of a torch and "it didn't look good" with pieces of wood everywhere, coconut fronds and fallen trees.
Bourke described herself as a tough cookie, but conceded that when the cyclone was roaring through, she was scared.
Makeshift evacuation centres will have hopefully provided safe haven for many residents, she says, with many warnings issued before the cyclone arrived.
A heavy rain warning is still in place, she says.
The last update from the Fiji Metservice at 1am was that the cyclone was still category 4, Martens told the Herald.
"The conditions are easing for Tonga, but it is not quite over yet."
Power had been out there since 7pm and there had been no observations since then, he said.
"The computer estimate was there could be 150-250mm of rain, maybe higher."
The cyclone was now now heading for the southern islands of Fiji, with it expected to arrive there later today.
"If it is not a category 5 by the time of the next update at 7am we expect it will be later today.
"The main islands won't be affected but the southern islands, which are low lying, could be badly affected.
Aid agencies warned coastal properties would be destroyed and livelihoods wiped out by the winds.
Police imposed a curfew from 9pm last night to 7am today in the central business district of the capital, Nuku'alofa, which is on Tongatapu.
As the cyclone bore down last night, Nuku'alofa resident Viliami Latu said the winds were getting stronger and stronger. Power had been shut off on Tongatapu, plunging everything into darkness.
He said at 10pm, with hours still to go until peak intensity, he and his family were bracing for what could be devastation.
Frederica Filipe said on Twitter her windows and doors were shaking as she could hear Cyclone Gita "roar". "At this stage the wind is blowing so hard that the rain is coming in under our doors."
Venaisi Tuilaga said on Facebook: "I'm here in Houma ... I just can't describe the wind and rain here."
Tooloo Charlies said: "Right now the wind is getting pretty bad, a mango tree has fallen into electric lines. Poorly built houses lost roofs."
The streets of downtown Nuku'alofa emptied before the curfew began at 9pm. Photo / Matangi Tonga
In a video posted on social media, several residents taking refuge in a Tongan church sang as they waited for the storm to hit.
Ariana Pupua, who posted the video, said they were "awaiting the cyclone with praise and worship".
"Jesus you are in control."
MetService meteorologist Bill Singh said Gita was still expected to develop into a category 5 cyclone, with sustained winds of 200km/h, and up to 250mm of rain.
The eye would pass about midnight 40km away from Tongatapu, which he said was "very close".
Singh said a storm surge from the sea could add to the devastation wind and rain would cause to low-lying and coastal areas.
New Zealand retirees John and Margaret Swift, who live in TeeKui on the main island, said at 6.30pm that they were as ready for the cyclone as they could be.
Their house was made of concrete blocks and was built for hurricane-force winds.
"The wind has been gaining in strength, there are still some people out on roofs nailing down and boarding up windows," Margaret said last night.
Their house was all run on electricity so they went into town to get some gas bottles but most stores were sold out. "We were able to borrow a gas bottle from a friend."
Another Kiwi, Joanna Burke, said she expected the nation's flimsily constructed homes to be devastated.
locals secured their homes as best they could. Photo / Taniela Hoponoa
Most are made of blocks, corrugated iron and wood, and she didn't expect them to fare well in the expected 200km/h winds.
New Zealander Richard Young, manager at Sandy Beach Resort on the outer islands of Ha'apai, said last night the worst he was expecting was winds up to 90km/h which was a stark contrast to the 200km/h winds expected on the main island.
"[However] we still moved all of our boats into safe locations, cleared the trees of coconuts, pulled shutters over exposed windows and any outdoor furniture is locked away.
"All of the guests have been given a briefing on what to do and when, and what to expect," Young said. All but three had left the resort.
Oxfam New Zealand was on standby to provide aid for up to "70 per cent of the country's population".
Deterioration of sanitation could raise the risk of a second disaster due to the spread of dengue and other vector- and water-borne diseases.
Prime Minister Jacinda Ardern said last night New Zealand was on standby to help. "We're ... staying in very close contact."
The cyclone is said to be the fiercest event ever to hit the country in its recorded history.
Hours before it came, locals were hammering planks of wood to windows and securing roofs.
The first winds hit last night.
Evacuation centres had been set up in nearly every village and people were able to shop for supplies again yesterday; Tonga's Sabbath laws meant many stores had to remain shut on Sunday.
Sione Taumoefolau of Tonga Red Cross said cyclones that have hit the country in previous years had mostly affected the outer islands.
"This is the first time for those people in Tongatapu to experience a very strong cyclone coming through Nuku'alofa," he said.
WeatherWatch said the cyclone was forecast to make "a direct hit" over the top of Tonga and the capital, Nuku'alofa, on the main island of Tongatapu late last night and early hours of today.
"Over 75,000 people are in the path of a storm already more powerful than anything New Zealand has ever seen, with torrential rain and damaging winds forecast to be averaging close to 200km/h and gusting over 230km/h by tonight [Monday] when it makes landfall there," a spokesman said.
Among New Zealand aid agencies on the ground was Tearfund, which was working closely with its partner association in Tonga, 'Ola Fou. Emergency packs for families were being provided and they were also equipping evacuation centres.
Over the weekend the cyclone caused flooding and wind damage in the neighbouring nation of Samoa.
A state of disaster was declared there as authorities focused on rescue and evacuation.
There were no reports of injury or death and emergency services were focusing on people isolated.
American Samoa's governor, Lolo Matalasi Moliga, said Gita had caused a lot of damage to homes and utilities there.
Singh of MetService said the weather system was more likely than not to reach New Zealand and if it did it would be an ex-tropical cyclone, but it was too early to be sure.
"The consensus of most of the models is it looks like it's going to turn towards us," he said. "The models are basically agreeing now that the system will move across central New Zealand from late Tuesday to early Wednesday. But it's still early stages."
