Fiji is struggling to clean up after Tropical Cyclone Winston slammed into the Pacific island nation at the weekend, bringing average winds of 230kmh.

Jeff Masters, Director of Meteorology at US-based Weather Underground, says the Category 5 cyclone was the “strongest storm” to have struck in the southern hemisphere.

“Damage to Fiji is going to be severe to catastrophic, but it will be several days before the true scope of the destruction is realised,” Dr Masters said.

Winston made landfall with sustained winds of about 297kph – the second-strongest by any cyclone worldwide. Super Typhoon Haiyan in the Philippines in 2013 remains the strongest, with sustained winds of about 305kph.

The death toll in Fiji has risen to 17, with at least 150 houses destroyed, according to latest updates from the Government and United Nations.

Aid agency Oxfam fears for the worst, because many areas hit by the storm remain cut off.

“The Fijians are desperately trying to repair severed lines of communication, but they hold grave fears that the news waiting for them will be dire,” Oxfam Pacific Regional Director Raijeli Nicole said today.

“Given the intensity of the storm and the images we have seen so far, there are strong concerns that the death toll won’t stop climbing today and that hundreds of people will have seen their homes and livelihoods completely destroyed.”

The UN Office for the Co-ordination of Humanitarian Affairs says the cyclone is one of the most severe to have hit the South Pacific.

“Power is out across most of the country,” it said. “Communications are down in many affected areas, particularly in the west, and this is slowing the collection of impact data.”

Foreign Affairs Minister Julie Bishop says Australia is dispatching an initial aid package of $5 million.

Canberra has also offered a P3 Orion aircraft to assist with aerial surveillance of affected areas, and MRH-90 helicopters to assess and provide relief to outlying islands.

“The full impact of this disaster is still not known,” Ms Bishop said. “We stand ready to provide further assistance to support Fiji’s relief and recovery efforts.”