Tropical Cyclone Pam triggered flooding along with evacuation orders affecting thousands of people in Vanuatu on Friday as it intensified to a maximum-strength storm offshore, officials said.

Aid agencies said many people living in flimsy slum accommodation were particularly at risk in the poor Pacific nation of 270,000 people, as well as those in remote outlying islands.

The Vanuatu Disaster Management Office issued red alerts for four provinces, advising thousands of residents to shelter in evacuation centres.

Meteorologists said Pam had hit the top category five intensity, with winds set to reach up to 230 kilometres per hour (143 miles per hour).

The cyclone was expected to track 100 kilometres off the island nation's east coast on Friday.

But Vanuatu's meteorology service said residents should still brace for winds of 165 kph (102 mph) along with flash flooding, landslides and "very rough to phenomenal seas".

UNICEF spokeswoman Alice Clements said Port Vila resembled a "ghost town" as people battened down.

"Tonight is really the night we're going to find out," the official with the UN children's agency told AFP.

"The winds have definitely increased, the palm trees are blowing around like crazy, you're starting to get that kind of howling wind coming through," she said.

Clements said there was little hope that the cyclone might make a late change of course and largely spare Vanuatu.

"They're super unpredictable but the centre of the storm is tracking really close by, so even if it's not a direct hit there's going to be really significant impacts," she said.

Save the Children's Vanuatu director Tom Skirrow said up to 50,000 children were at risk in the nation, where two-thirds of the population rely on subsistence agriculture.

"We have been going door to door in some of the poorest slum areas and I'm hugely concerned that not enough is being done to make sure children and families are safe as this huge storm approaches," he said.

"Thousands of families are living in makeshift, flimsy houses which will not withstand the immense winds and rain we're expecting. Families need to urgently evacuate to safe buildings or the results could be catastrophic."

Meteorologist Neville Koop from Fiji's Nadraki Weather Service said Pam's winds were capable of bringing down even well-built structures.

He said they could be more destructive than Cyclone Uma in 1987, which killed at least 30 people when it sank two ferries off Port Vila.

"Pam has winds which are much stronger than Vanuatu experienced" in 1987, he said.

Australian Foreign Minister Julie Bishop issued a statement urging citizens "to reconsider your need to travel" to Vanuatu.

"We are monitoring the humanitarian impact closely and are preparing a potential response to this emergency," Bishop said.

Cyclones, known as hurricanes in the northern hemisphere, are a common occurrence in the South Pacific at this time of year.

Tropical Cyclone Lusi swept across Vanuatu in March last year, killing 11 people and damaging crops and infrastructure.