At least six people have died after a magnitude-7.3 earthquake struck near Vanuatu's capital city of Port Vila, according to local media and the United Nations.

An official at Port Vila's hospital told Vanuatu's national broadcaster VBTC that more than 50 were injured.

Australia is sending urgent assistance to the South Pacific island nation where the death toll is expected to rise as search and rescue teams work to reach people trapped under collapsed buildings.

Australian Foreign Minister Penny Wong said the earthquake caused "significant damage", adding that Australia would send urban search and rescue teams, as well as emergency medical assistance, to help recovery efforts.

"We are closely monitoring the situation and stand ready to provide further assistance to the people of Vanuatu as the extent of damage becomes clear," Senator Wong said in a joint statement with Matt Keogh, the acting minister for international development.

"Australia and Vanuatu share a deep and enduring partnership. We are family and we will always be there in times of need."

Local business owner Michael Thompson described "chaotic" scenes in Port Vila overnight as rescuers searched for people believed to be trapped under crushed buildings.

"We understand there are eight people trapped inside and the guys are trying to move slowly to get in," he said.

Hospital 'struggling to cope'

Clement Chipokolo from World Vision Vanuatu told the ABC's The World program that he expects the death toll from the earthquake to rise due to the severity of the damage.

"There's quite significant damage that has occurred. We observed as we drove around a number of building that were flattened completely. So we imagine that there are still some people that are under those buildings," Mr Chipokolo said.

"There is an active search and rescue mission that is ongoing at the moment.

Mr Chipokolo said damage to critical infrastructure such as electricity and phone lines was hampering the recovery efforts.

"Lights are completely out … We don't have water across the city, and most of our communications systems are down," Mr Chipokolo said.

"We anticipate that the number of deaths will continue to go up, given the number of people that are being treated as casualties."

Locals describe 'mass casualty event'

Glen Craig, chair of the Vanuatu Business Resilience Council, described the destruction he had seen from his car across the city.

"At this stage, we think there's around 10 buildings that have collapsed around the Port Vila area," he said.

He warned the situation was likely to be a "mass casualty event" with several retail buildings destroyed around the airport, wharf area and downtown, and communication infrastructure also damaged.

"We've seen deceased pulled out of downtown retail, like the Billabong building," he said.

"We don't know if [the main communications cable] was damaged, but we know there was damage to the cable centre at Miller Road."

The main telecommunications provider hoped to bring the network back up and running "in a couple of hours", he added.

Elsewhere in Port Vila, Mr Craig saw vehicles crushed by the impact of falling debris.

He said international assistance, specifically engineers and search and rescue teams, was urgently needed.

Mr Craig said stranded tourists had relocated to safer spaces, including diplomatic missions.

"There are at least 50 to 100 tourists up at the Australian High Commission," he said.

Half a dozen aftershocks hit Vanuatu following the initial quake, data from the US Geological Survey showed.

Several were heavy enough to be felt in Port Vila, local journalist Dan McGarry said.

"There was one that had us all sort of perched and ready to move again. But none of them have been anywhere near as bad as the initial shock," he said.

Mr McGarry described some buildings as being "pancaked".

"There are a great many walls down, there is one building that I've seen for certain that's collapsed in the middle of town — it's pancaked down," he said.

"There are cracks visible in numerous buildings around town, old and new. Everybody's going to have to do a bit of a stocktake."

Vanuatu is ranked as one of the countries most susceptible to natural disasters such as earthquakes, storm damage, flooding and tsunamis, according to the annual World Risk Report.