The people of Tonga are preparing for what's being described as the strongest cyclone to ever hit the island kingdom.

It's a situation being made more stressful by the uncertainty of what Tropical Cyclone Gita will bring and efforts to prepare being hampered by religious laws closing trading on Sundays.

Gita is expected to hit the tiny country sometime over Monday night or Tuesday morning. About 11am Monday, a state of emergency was declared ahead of the category 5 cyclone's arrival.

COCO NEW THE AGENCY Nastajia Bourke's sister Joanna's house in Umusi boarded up ahead of Cyclone Gita hitting Tonga.

Power was out for much of the island by 9pm, and Kiwis were desperately trying to keep in touch with family members in Tonga whose cell phone batteries were dying.

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Wellingtonian Nastajia Bourke lost contact with her 43-year-old sister Joanna — who was alone in the family's home at Umusi — at around 11pm and was anxious about how she was faring.

"The last we heard was her saying the roof was starting to lift, that water was coming in through her roof and the front door, which must have blown open," she said.

Joanna's last tweet was around 10pm, and noted the cyclone was "getting mad".

NTCWC Tropical Cyclone Gita is expected to head west for several days, hitting Tonga overnight and strengthening to a category 5 storm.

No sugarcoating folks - it’s pretty frightening outside with the wind - she’s consistent but the large swells are quite scary - she’s getting mad 😡 #CycloneGita — Joanna Bourke (@CocoNewTONGA) February 12, 2018

Joanna had been holed up with her dog, said Bourke, but the dog had run off in the storm and it was too dangerous to follow him outside.

She had travelled to Tonga, where she is based for part of the year, from New Zealand just two days earlier.

TONGA GOVERNMENT The declaration of a state of emergency by the Tongan Government as Tropical Cyclone Gita approaches the country.

Bourke said her sister had barricaded the house on the advice of her builder brother, and packed an emergency evacuation bag in preparation for the storm.

"We'd all been talking non-stop on the group family chat before the storm, giving her comfort since she's over there alone," she said.

"She was doing well. But once the power goes out, your emotions change and the fear sets in."

@hannarosebutler/TWITTER The Samoa Red Cross team assessing the damage from Cyclone Gita.

Joanna's house was relatively modern and built to withstand harsh weather; as even that was letting in the storm, Nastajia said she was concerned for people in simpler homes.

"They will most likely lose everything, including their plantations — which are their bread and butter," she said.

TVNZ correspondent Barbara Dreaver, who was in Tonga, tweeted at around 11pm that her hotel was shaking in the wind and rain was "coming in sideways".

STUFF Residents in Tonga are bracing themselves for the impending category 4-strength Cyclone Gita.

Hotel is shaking and can hear screech of metal flying past outside.. rain water coming in sideways to second floor room. Heart with those not in solid structure this is bad gita #tonga — Barbara Dreaver (@barbaradreaver) February 12, 2018

The Ministry of Foreign Affairs and Trade has advised against "all tourist and other non-essential travel" to Tonga.

GOOGLE Tagata Pasifika reporter John Pulu is staying in the Tanoa International Dateline Hotel in Nuku'alofa - right on the waterfront.

Tagata Pasifika reporter John Pulu is in Tonga's capital, Nuku'alofa, staying in the Tanoa International Dateline Hotel - right on the waterfront.

He was at a press conference on Monday morning when the state of emergency was announced.

"At the moment ... Tonga is in preparation for what is being described as the strongest cyclone to ever hit [the country]," Pulu said.

@hannarosebutler/TWITTER The Samoa Red Cross team assessing the damage from Cyclone Gita.

The fact it was likely to hit overnight - combined with the potential for it to knock out power and communications networks - made it even more frightening.

"We don't know how strong the cyclone is going to be. That's the pain of all of this."

With the powerful storm came "potentially a loss of human life and huge damage to buildings", Pulu said.

"The advice from the minister in charge of the emergency department is that everyone go home and prepare."

Many Tongans were turning to prayer to help them through the storm.

The MUST Be Tonga Facebook page posted a video of girls and women singing: "Praise & Worship is the shield from the storm," admin wrote.

People were told to seek shelter in Mormon churches and school halls present in most villages as they were the most robust structures.

There were also instructions to keep track of where people had gone in the case of fatalities.

But efforts to prepare were hamstrung on Sunday due to a religious ban on trading.

One hardware store opened for a short time, with people travelling from the other side of the island to stock up, only for police to close the shop, Pulu said.

When asked if the rules preventing trade on a Sunday were frustrating given the situation, Pulu said: "absolutely."

Making an exception in emergencies was an issue to consider after the cyclone passed.

Among the infrastructure that could be affected was Nuku'alofa's central business district and the royal palace - both located on the waterfront.

Low-lying and coastal villages were also threatened.

Twitter user Alex Duncan tweeted a video of palm trees thrashing in the powerful wind earlier in the evening.

The wind is really starting to pick up in #Tonga I hope everyone is staying safe. #CycloneGita pic.twitter.com/UjM6J3WENk — Alex Duncan (@Durzo159) February 12, 2018

Pulu said while some of the newer buildings were well-constructed, a lot of the housing was traditional and not likely to hold up well.

Some stores and houses were being boarded up ahead of Gita's arrival.

Regardless of the efforts, Pulu had his doubts.

"I don't think the country is quite prepared for something that is supposed to be the strongest on record.

"I think some [people] seem to have that island mentality of 'we've been through this before, we know what it's like'. I don't know if they are prepared..."

At 7.45pm on Monday, it was almost dark outside and Pulu said the wind and rain were becoming significantly stronger.

"Police are driving around Nuku'alofa, especially the waterfront, trying to warn people to go home," he said.

Health and safety was an issue, Pulu said, with "people still jumping into the ocean for a quick dip".

The Tongan police commissioner issued a curfew from 9pm Monday until 7am Tuesday to ensure no-one left the capital.

Power would be cut just before the cyclone hit to minimise the risk of people getting electrocuted, Pulu said.

"We're surprised the power is still on here at the hotel. I'm looking outside and power in the homes is still operating, the street lights are operating," he said.

"Trees are going to go flying and debris will go flying into the wires."

Emil Adams tweeted a photo of tangled branches that had washed up on Apia's shore, beneath a tree bent back in the wind.

debris washed down from Vaisigano River a spectacle on the Apia waterfront #cyclonegita pic.twitter.com/ERcAz2Oejn — Emil Adams (@tamaligi) February 12, 2018

To make matters worse, there's been an outbreak of dengue fever in Tonga this summer.

There had been 50 cases and one death, Pulu said. That death was 12-year-old Auckland girl, Toafei Telefoni. She'd been on a family holiday.

She died in hospital on January 24, six days before she had been due to arrive home in New Zealand.

Pulu's hotel manager told him New Zealand was on standby, as was the Australian government.

New Zealand Prime Minister Jacinda Ardern said the Government was prepared to engage with any Pacific Island nations affected by the cyclone.

"My latest advice is that we haven't been asked for direct assistance from Samoa as yet, as a result of the damage that they've received. But we are on standby, ready and waiting, to discuss with them any needs that they might have," she said.

"The same, of course, would go for Tonga. We're waiting to see the impact there but staying in very close contact.

"Niue, my understanding is that they did not bear as much of the brunt as was first anticipated that they might."

At 9pm on Monday, Fiji-based consulting meteorologist at Na Draki Neville Koop said the cyclone had been upgraded to a category 5 and would likely hit at about midnight.

"The eye is going to pass very close to, if not directly over, Tongatapu, the island where the capital is," he said.

"It is potentially a very destructive storm. The wind damage is going to be quite significant."

Some Fijian islands, which Koop described as "not particularly populated", were also under cyclone warning.

MetService tropical cyclone forecaster Micky Malivuk said its track on Monday morning suggested it would pass close to Tongatapu. The centre would probably pass just to the south, with the area just to the north exposed to the strongest winds.

"If you're just a little bit away from the centre, that's where the strongest winds are," Malivuk said. Tongatapu would be "pretty much exposed to the full brunt of the cyclone".

Gita was starting to form an eye. "It just implies it's becoming a very intense storm when you have an eye forming."

New Zealanders in Tonga who require consular assistance should contact the New Zealand High Commission in Nuku'alofa, Tonga on +676 23122, the after hours number of +676 881 7022 or at nzhctonga@mfat.govt.nz.