The The first images have begun to emerge of the destruction on Tanna, about 200 kilometres south of the capital Port Vila, as aid workers and officials begin to assess the damage on the island.
Follow our reporter in Port Vila
Tanna and its 29,000 people took the full force of the category five storm, which carried winds of up to 300 kilometres an hour when it tore through Vanuatu last week.
The Australian Red Cross said most of the homes on the island were reported to have been destroyed.
Aid agencies and rescue teams from New Zealand and Australia had flown over the islands, but have so far been unable to land because of flooding.
Authorities have struggled to contact the islands that bore the brunt of Cyclone Pam, which flattened buildings, smashed boats and washed away roads and bridges.
A New Zealander who made it off the island said the devastation there was simply indescribable - and worse than Port Vila where thousands were homeless.
Mike Keegan was in Tanna for a holiday and when he first arrived on Wednesday was told the storm would miss the island.
By Saturday morning he was taking cover in the passage way of a concrete guest house while Cyclone Pam roared outside.
"It sounded like a 747 parked on the roof. Just a horrendous noise, just horrendous."
Red Cross regional delegation head Aurelia Balpe said because of the communication black-out on Tanna, the agency did not know if emergency supplies on the island had survived the storm, or had been distributed.
Ms Balpe said getting aid to the island may not be straightforward.
"I know that the airport was damaged in the cyclone. Small planes were able to land over the last couple of days, but there would need to be some kind of assessment of the viability of that airstrip for bigger supply planes.
"And I am assuming the assessment today will be looking into that."
Death toll at 24
The United Nations has put the death toll from the cyclone at 24.
"There are 24 confirmed fatalities, 11 from Tafea, eight from Efate and five from Tanna," the UN's Office for the Co-ordination of Humanitarian Affairs said in a situation report.
But there were discrepancies with the figure, as the National Disaster Management Office has put the death toll at 11 - five on Tanna Island and six in Port Vila.
The fatalities included a teenage boy and his mother, killed by flying debris, and two women crushed by a wall of a church they were sheltering in.
The National Disaster Management Office said an expatriate died when he went down with his yacht, but has withheld his nationality.
Up to 10,000 people in the capital Port Vila alone were homeless.
Tanna buildings 'ravaged'
Aid workers who have managed to land on Tanna Island say almost all of the traditional houses have been destroyed and residents are in desperate need of food and water.
National director of World Vision in Vanuatu Michael Wolfe visited the island yesterday and said only one of its general stores survived the cyclone and was still stocked.
"All the other stores have been ravaged and the roofs are blown off, or the windows have blown out. Some of the buildings are still standing but it's just a skeleton of cement block."
Mr Wolfe said there appeared to be enough food for about five days, and it would take about four months for local crops to grow back.
The Red Cross has also managed to reach the island and said it had been devastated.
'It's like a war zone'
Radio New Zealand International reporter Koroi Hawkins, who arrived in Port Vila late last night on a New Zealand Defence Force Hercules, said the capital was unrecognisable.
"Everything seems to be touched by destruction," he told Morning Report. "I saw a huge warehouse made of brick, the wall has been split in two down the middle by the sheer force of the wind."
"It's not like it's torn through just part of the town - everything is looking dead twisted or broken," he said. "It's like a weird Pacific winter ... if anyone's been to Port Vila, this lovely French-looking idyllic little town, it's like a war zone."
Mr Hawkins said Red Cross relief supplies do not appear to have been distributed yet, as supplies were being stored and assessed before being distributed to the people to ensure it was handed out equally.
"None of the relief supplies, from the Red Cross at least, are going out yet. They are waiting for assessments, waiting to know what the whole situation is before they break out."
Mr Hawkins said there has been some reports of looting in Port Vila.
"So there is some opportunists going around... because you see it's totally dark in a lot of parts of the city. There's only some places that have only yesterday afternoon had their power switched on."
Ni-Vanuatu are faced with great uncertainty over their future.
The deputy principal of a Port Vila school that's being used as an evacuation centre sheltering over 500 people said it's not clear how long they could stay there.
Sikal Iuarel said there was enough supplies for residents living at Fresh Wota School, but more clean drinking water will be needed soon.
Mr Iuarel said frustration is growing among the evacuees about their uncertainty.
"People who are here just want to know if the government could give some answers to them quickly," he said.
"They're living in an institution and school could resume at any time, and they're rebuilding, and if they government could come out publicly and tell them 'yeah, you go back home and we'll provide you tents and tarpaulins'."
Sikal Iuarel said the government says schools will be closed for at least the next week.
'Substantial slump' likely for economy
Vanuatu's president Baldwin Lonsdale said his devastated country needs immediate help, as Cyclone Pam had wiped out all development of recent years and his country would have to rebuild everything.
An economist says it may take years for parts of the Vanuatu economy to recover after Cyclone Pam.
The Asian Development Bank's Milovan Lucich said Vanuatu had enjoyed more than a decade of growth but was looking at a substantial slump as a result of the cyclone.
Tourism had been one of the country's largest contributors to growth, making up around 25 percent of gross domestic product.
However Mr Lucich said it would be some time before it can return to normal.
"It will take a while to get back to pre-cyclone levels of activity. In the experience of other natural disasters in other tourist hubs they've generally seen it generally takes one to two years for tourism to fully recover."
Measles outbreak feared
Aid workers fear a measles outbreak in Vanuatu could tighten its grip in the aftermath of Cyclone Pam, as thousands of people live in close confines in evacuation centres.
Unicef spokesperson Alice Clements said Vanuatu was already experiencing a measles outbreak and the agency was worried it would spread in post-disaster conditions.
"There are six vaccination teams going around the community in Port Vila right now working to immunise children and communities against measles.
"Because at the moment there is a measles epidemic and people are now crushed together in their shelters, small spaces, confined spaces, which is a great way to transmit diseases like measles."
Ms Clements said Unicef was also trying to get fresh water to communities to prevent the spread of water-borne diseases.
Hospital to become fully operational again
Vanuatu's Disaster Management Office said the country's main hospital in Port Vila would start to become fully operational again, after it sustained substantial damage in tropical cyclone Pam.
Mr Garae said authorities have spent the past few days clearing debris and cleaning up the damaged wards so they can be brought back into use.
Electricity and Water has also been restored to the hospital.
Mr Garae said Australia and France had also provided medical assistance to help the hospital.
Regional countries extend more relief support
Papua New Guinea and Fiji are the latest regional countries to offer relief and aid assistance to Vanuatu following the devastation of Cyclone Pam.
Late Tuesday, Fiji was sending a relief team to Vanuatu to assist in disaster recovery efforts.
The relief flight was also carrying basic supplies.
PNG pledged 1.8 million US dollars to Vanuatu for immediate disaster relief and recovery funding, as well as around 400-thousand US dollars to Tuvalu following the storm surge damages caused by Pam.
Earlier, the Australian government announced it would provide additional medical assistance and urban search and rescue capability for the cyclone victims in Vanuatu.
The foreign minister Julie Bishop said the Urban Search and Rescue, or USAR, team of 54 personnel, which arrived in Port Vila todaym would help clean-up and repair the Port Vila Hospital Complex.
The USAR team would also assist the Vanuatu government to assess the damage and potential recovery needs of other major infrastructure in Port Vila.
The French territories in the Pacific have also been sending aid to Vanuatu, as part of the FRANZ agreement between France Australia, and New Zealand.
A French military plane travelled to Port Vila on Monday carrying tarpaulins to shelter a hundred families, as well as chainsaws, a satellite unit, a logistics unit and seven emergency staff.