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The destruction caused by Cyclone Pam "could potentially be one of the worst in Pacific history", Unicef has said.
Unicef New Zealand executive director Vivien Maidaborn said: "While it is too early to say for certain, early reports are indicating that this weather disaster could potentially be one of the worst in Pacific history.
"The sheer force of the storm, combined with communities just not set up to withstand it, could have devastating results for thousands across the region."
Dozens of people are feared dead after Cyclone Pam pummelled Vanuatu overnight, with those on the ground describing the "utter devastation" it has caused in the capital.
There are no official reports of deaths or injuries, but there is an unconfirmed report that 44 people have died in Penama Province, the United Nations Office for the Coordination of Humanitarian Affairs (UN OCHA) said.
It said the regions of Maewo, Ambae and Pentecost were most likely to have been hit the hardest, with an estimated 30,800 living in those areas.
The category 5 storm was travelling in the direction of the most populated island of Efate, where more than 65,000 people live, including the capital Port Vila.
"The southern islands (total population 32,540) are also likely to be directly hit," the agency said.
"The Pacific Humanitarian Team is preparing to support the government-led response to the cyclone, and continues to preposition staff and stocks both in country and within the region."The UN aid agency said the tropical cyclone had moved westward of its expected track, placing several islands of the Vanuatu archipelago "directly in the path of the very destructive eye region of this cyclone".
"TC Pam is near peak intensity with winds in the eye region averaging 130 to 140 knots (250 to 270 km/h) with gusts up to 180 knots (340 km/h)," it said in its most recent update.
"This is an extremely destructive cyclone and those in its path will be in great danger. The expectation is that the western edge of the eye will pass over or extremely close to the eastern side of Efate (where the capital Port Vila is located) between 10pm and midnight local time in Vanuatu on 13 March."
Care International has spoken with it's project manager on the ground in Port Vila and heard of the "utter devastation" in the capital.
Media advisor Dylan Quinnell said project manager Charlie Damon described to him what she found when she emerged from her shelter this morning.
"The winds had died down a bit and basically what they've found is a scene of utter devastation.
"The trees are down all around the hotel where they were sheltering. Charlie said to me she thinks it would be most of the day just to clear the trees around the hotel for vehicle access.
Ms Damon said she had been told some of the evacuation centres providing shelter for people had lost roofs and flooded in the storm.
Mr Quinnell said he understood initial reports were dozens of people may have lost their lives but there was no way to verify that yet.
MetService said the storm had intensified overnight and was was expected to get even stronger during the course of the day.
At 1am NZT the severe tropical storm was located around 70 kilometres east of Port Vila, the capital of Vanuatu.
The category 5 cyclone "intensified slightly overnight", MetService said, with the central pressure dropping to an estimated 899hPa and winds close to the centre of about 250km/h.
"The area of gales (65km/h winds) is estimated to extend up to 380km from the centre of the cyclone, affecting much of Vanuatu," MetService said in a 5.20am update.
"TC Pam is still moving slowly south, and is expected to cross the southern Vanuatu islands of Erromango and Tanna during the next 12 hours while intensifying slightly."
The tropical cyclone is still on track to skirt the northeast of New Zealand over the coming days, bringing with it heavy rain, severe gales and high seas.
A New Zealander who spent the night hunkering down from the storm, described herself as "one of the lucky ones" in an email to NZME. News Service overnight, because she was sheltering in "a solid hotel" in Port Vila.
Kristy Norton, who moved to the island capital more than two years ago with her husband and two children from Queenstown, said she felt "very safe here and well looked after" by staff at the Grand Hotel.
"It's now 8.25pm local time [10.30pm NZT] and Cyclone Pam is on top of us and appears to be slowing down her tracking speed while she's here in Port Vila," she wrote.
"Local hotels and businesses have opened their arms and doors to local people encouraging them to find strong shelter as their shanties and small village homes have no chance of survival. I sit here typing to you and it's the noise that's the worst."
There had already been reports of loss of water and power at the time of writing, she said.
"It's hard to comprehend the devastation that we'll wake to tomorrow. The stories that people are reporting on social media frighten me to tears...make me sick to my stomach," she said.
"There'll be loads of broken homes to repair tomorrow and many broken hearts that may never repair."
The Ministry of Foreign Affairs and Trade has issued a travel advisory warning New Zealanders against all tourist and non-essential travel to Vanuatu.
Cyclone Pam had passed close to the southern islands "bringing high winds, heavy rainfall, flooding and damage to infrastructure", it said.
"Communications infrastructure has been damaged by the cyclone and it could take some time for communication lines to be restored in the worst affected areas," Mfat said, adding that both telephone and internet connections were down.
"Flights to and from Vanuatu are affected by the cyclone and Vanuatu's international airport is currently closed. It is anticipated commercial flights will continue once the cyclone has passed through and the airport reopens.
"New Zealanders in Vanuatu wishing to depart are advised to contact their airline, travel agent or travel insurance provider directly to make arrangements."
Mfat advised Kiwis in Vanuatu, Fiji and New Caledonia to register their details on SafeTravel, and keep family and friends in New Zealand updated.
* If you have concerns about a New Zealand citizen family member in Vanuatu, try to make direct contact in the first instance, and if you have ongoing concerns, contact the Ministry of Foreign Affairs and Trade on: 04 439 8000.
* New Zealanders affected by Tropical Cyclone Pam who require consular assistance are advised to contact the relevant New Zealand High Commission or Consulate-General:
The New Zealand High Commission Port Vila, Vanuatu - 678 22 933 or email@example.com (Please note that the High Commission in Port Vila has closed to the public)
The New Zealand High Commission Suva, Fiji - +679 331 1422 orNZHC@unwired.com.fj
The New Zealand Consulate-General Noumea, New Caledonia - +687 272 543, +687 79 19 22 or firstname.lastname@example.org.
By Patrice Dougan of NZME. News Service