A communications blackout is preventing aid workers from assessing the scale of the damage in cyclone-stricken Vanuatu.
Tropical Cyclone Pam hit the South Pacific island chain on Friday, bringing flooding, high seas and winds in excess of 250km/h - one of the worst storms in the island nation's history.
While the destructive system has knocked out communications in much of the region, there are unconfirmed reports more than 40 people are dead.
"A lot of people would have spent the night in their homes in the hope that everything would be ok, but there has been significant destruction of homes in some areas," Oxfam country Vanuatu director Colin Collett Van Rooyen told AAP.
"We do know there have been some casualties.
"We do anticipate that there'll be higher numbers (of casualties) on remote islands, because they have less sophisticated shelter, but we have no indication of what's happening there because we have no comms to the islands."
Speaking from Port Villa, Mr Collet Van Rooyen said people were still moving into evacuation centres since the storm hit.
"What's about to happen is going to be a significantly sized humanitarian response, but it will probably take months if not years," he said.
Foreign Minister Julie Bishop says Australia, which is Vanuatu's principal aid donor, is ready to help.
"We are still assessing the situation but we stand ready to assist," she said on Saturday.
"We stand ready to support with whatever is needed, working with our partners, New Zealand and other countries in the Pacific."
She said there were probably 3000 Australians in Vanuatu at any one time, but the government had not received any reports regarding Australian citizens.
The storm crossed the main Vanuatu island where more than 65,000 people live and a group of islands further south, which are home to 33,000 people.
Some locals on the southern island of Erromango reportedly took shelter in caves as the severe conditions intensified.
"It's a traditional coping strategy," Red Cross worker Aurelia Balpe said.
As the storm hit, World Vision worker Chloe Morrison spent a terrifying night waiting for the category five system to pass through.
"There are reports from our other colleagues of entire villages being literally blown away overnight," she told AAP.
UN Secretary General Ban Ki-moon said he had met the president of Vanuatu on Saturday morning and conveyed "our deepest condolences" as well as solidarity with the people of the archipelago.
New Zealand has pledged an initial $1 million to help cyclone-stricken Pacific nations and an RNZAF P3 Orion is in the air assessing the damage.
The Australian Red Cross has launched an appeal to help the many thousands of people affected by the cyclone.
