The first reports from the outer islands of Vanuatu on Monday painted a picture of utter destruction after a monster cyclone tore through the Pacific island nation.

Authorities in the South Pacific nation were struggling to establish contact with the islands that bore the brunt of Cyclone Pam's winds of more than 300 kph (185 mph), which flattened buildings, smashed boats and washed away roads and bridges as it struck late on Friday and into Saturday.

The government's official toll is eight dead and 20 injured but that looks certain to rise, given the extent of the damage.

The southern island of Tanna, about 200 km (125 miles) south of the capital, Port Vila with its 29,000 inhabitants, took the full force of the category 5 storm. Early reports said it had been devastated.

"What they saw was lots of debris, the foliage was completely destroyed, lots of uprooted trees," the Red Cross regional head Aurelia Balpe told Radio New Zealand, citing a pilot who had been able to land on the island.

"All corrugated iron structures destroyed, concrete structures, all of them without roofs."

There were unconfirmed reports of two deaths on the island, but the toll is expected to rise significantly.

Balpe said the main town on the island of Erromango looked to have been "flattened".

In Port Vila, where seas were reported to have surged as high as 8 metres (26 feet), a clean-up was under way with as much as three-quarters of the capital's houses reported destroyed or severely damaged.

"It just looks like a bomb has gone off in the centre of town," said Alice Clements of Unicef from Port Vila.

"The trees are shredded, there are power lines everywhere, the corrugated iron has been wrapped into strange shapes by the force of the wind."

Aid arriving

She said communities would have to rely on fallen fruit and root crops in the short term and they faced the prospect of no water, no food and no power.

Military flights from New Zealand and Australia were bringing in water, sanitation kits, medicines and temporary shelters for the estimated 10,000 made homeless on the main island.

Commercial flights were also due to resume on Monday to bring in more aid and take out tourists.

Formerly known as the New Hebrides, Vanuatu is a sprawling cluster of 83 islands and 260,000 people, 2,000 km (1,250 miles) northeast of the Australian city of Brisbane.

It is among the world's poorest countries and highly prone to disasters such as earthquakes, tsunamis and storms.

Aid officials said the storm was comparable in strength to Typhoon Haiyan, which hit the Philippines in 2013 and killed more than 6,000 people, and looked set to be one of the worst natural disasters the Pacific region has experienced.

Australia promised A$5 million in aid, New Zealand NZ$2.5 million while Britain, which jointly ruled Vanuatu with France until independence in 1980, has offered up to two million pounds ($2.95 million) in assistance.

"We have made a substantial start but of course it's early days so we have to assess the damage," Australia's foreign minister Julie Bishop told ABC TV.

"This will be an ongoing operation over a number of days, weeks, months ahead."

The World Bank said it was exploring a swift insurance payout to the government, and the International Monetary Fund said it was ready to send funds and assistance to rebuild the economy.

Pam was losing its intensity as it passed by the east coast of New Zealand's North Island, where it was bringing strong winds, rain, and big seas but so far little damage.