Whole villages in Vanuatu are reported to have been blown away by Cyclone Pam, with unconfirmed deaths being put at 44.
Eight deaths have been confirmed in Port Vila, home to about 40,000 people, but aid agencies say the death toll is likely to climb.
The official death toll stands at six, but authorities say it is likely to be significantly higher.
The worst cyclone in 12 years has left devastation in the capital, with disaster officials saying 80 per cent of the houses have been seriously damaged.
Government offices and other infrastructure have been destroyed, along with part of the main Vila Central Hospital.
Tom Skirrow, of Save the Children, said people were wandering the streets looking for help.
How bad things are in the northern islands is not known because all communications have been cut.
And there is no contact with Tanna island in the south which the cyclone is pounding now. Twenty thousand people live on Tanna.
Emergency services in Vanuatu are preparing to mount a massive search and rescue operation in the aftermath of Cyclone Pam.
Spokesperson for the National Disaster Management Office, Mishaen Lulu Garae, said the capital Port Vila had been completely destroyed.
He said communication with the northern provinces was impossible, but he assumed the area was devastated and it could take days to get damage assessments from those areas.
Vanuatu's council of ministers held an emergency meeting this afternoon and international assistance is likely to be requested.
Cyclone Pam reportedly generated gusts of up to 330 kms/hr and Metservice said it was the worst Pacific storm since cyclone Zoe in 2003.
The gales extended up to 380 kilometres from the centre of the storm - slightly longer than the distance between Wellington and Taupo.
NZ makes initial $1 million donation to region
New Zealand will contribute $1 million in initial funding to Pacific nations battered by the cyclone
The funding includes $200,000 in response to specific requests for help in Vanuatu, Fiji, Tuvalu and Solomon Islands.
NGOs in Vanuatu and Solomon Islands will get $400,000 to help with their aid efforts.
Foreign Affairs Minister Murray McCully said an Air Force Orion is already in the region, conducting a damage assessment flight over Tuvalu.
He said the government will continue to watch the situation and will give more aid if needed.
UN regional director Sune Gudnitz, who is in Fiji, said the immediate concern was that there will be a very high death toll in Vanuatu but also an enormous amount of destruction and devastation.
Unicef spokesperson Alice Clements, who is in Vanuatu, said the storm had gone on far longer than any one expected and it was terrifying.
"I stayed in a concrete hotel that was three storeys high, and even so I'd lost the sliding doors from my room, I had all the wind howling through the room."
She said the wind was still incredibly strong, but not as strong as it was at midnight (local time).
"It was 15-30 minutes of absolute terror for everybody in this country."
She said there had already been reports of some deaths.
"We have some very unconfirmed reports of casualties from the outer islands as well but we're waiting to get official confirmation on those, which is very sad news if it's true."
Extensive damage reported in other Pacific countries
In the other Pacific countries swiped by Cyclone Pam, extensive damage is being reported from some islands but contact with others has been lost.
A seven day State of Emergency has been declared in Tuvalu and the general election due on to be held next Thursday, has been delayed.
The Red Cross said the outer islands had been the worst affected but it was unable to make contact with them.
In Solomon Islands, Tikopia is one of the worst affected islands after Pam passed directly over it.
Disaster officials said the island was like a desert, with most of the food gardens gone.
Malaita Province has reported considerable damage with authorities there saying houses have been flattened.
In New Caledonia, Cyclone Pam is expected to pass 160 kilometres east of Mare, in the Loyalty Islands, this afternoon.
The territory's government has ordered all people to stay indoors until further notice.
And authorities in Fiji said they are continuing to monitor Pam and people have been urged to take precautions and stay up to date with weather forecasts .
Kiwi tells of 'white noise' in Port Vila
Expat New Zealander, Jeff Brown who had been sheltering in a concrete motel on the outskirts of Port Vila, said the roof ripped off his family's unit overnight, and forced them to flee.
Mr Brown said the worst of the storm had now passed, but at its height early this morning, the sound of the wind and rain was like very loud white noise.
"My house - I built it myself - I'm not sure it will withstand the force of the wind that is going to come, so we've moved into a motel in town which is a concrete structure."
Mr Brown said he could now see outside and the area around the motel is "one hell of a mess", with debris everywhere.
He said the motel was sheltered against a bank and others in lighter accommodation will have lost everything.
A spokesperson for the Vanuatu National Disaster Management Office said Pam was expected to be worse than Cyclone Uma in 1987 - which killed 50 people.
A state of emergency has been declared in Tuvalu after tidal surges caused by Cyclone Pam to the southwest washed away houses and crops.