Fiji and Tonga issued severe weather warnings Tuesday as twin cyclones intensified near the Pacific island nations, with Fijian  officials preparing evacuation centres amid predictions a category-five super-storm is looming.

The Tonga Metservice said Tropical Cyclone Zazu was a category one system, currently packing winds of up to 90 kilometres an hour (56 miles an hour), and expected to reach category two on Wednesday.

Meanwhile, forecasters in Fiji said Tropical Cyclone Yasa had reached category three status as it rumbled about 600 kilometres off the coast of the main island Viti Levu, and could top the scale at category five by late Thursday.

Authorities in both countries advised residents to prepare for strong winds and flash floods, with Fiji Prime Minister Frank Bainimarama urging his people to prepare evacuation plans.

"We should all prepare now for heavy rain, damaging winds, coastal inundation, and flooding across the country," he said.

Bainimarama said Fijians should board up their homes, prepare emergency supplies and familiarise themselves with the location of their nearest evacuation centre.

The New Zealand-based meteorological service Weatherwatch said the twin cyclones were unlikely to join up but their proximity to one another did complicate forecasting.

"It makes it a little tricky to know exactly how close Yasa will get to Fiji -- direct hit is the current thinking," Weatherwatch managing director Philip Duncan said.

The storms are the first to form in the South Pacific's current cyclone system, which runs until May next year.

The region was battered by Tropical Cyclone Harold in April this year, a category five storm that gouged a trail of destruction across the Solomon Islands, Vanuatu, Fiji and Tonga.

Such intense cyclones were once rare but have become increasingly common in recent years, with Bainimarama among those blaming the phenomenon on climate change.

He said the island nation of about 900,000 people must again prepare for a worst-case scenario, just eight months after it last endured a lashing from a super-storm.

"Let's remember Cyclone Harold -- at the last minute it ramped up in strength and ended up being worse than predicted," he warned.

"Do not be caught off guard by this latest storm."