Category five tropical cyclone Winston is bearing down on Fiji.

Air New Zealand and the Flight Centre are advising customers to reconsider their travel plans as Tropical Cyclone Winston bears down on Tonga and Fiji.

The category four storm performed a u-turn over the South Pacific and is now heading towards Fiji.

The airline said flights to and from Nadi, Fiji could be affected by the storm, which had increased in strength.

"Air New Zealand is recommending customers with existing bookings for flights to Fiji this weekend reconsider their plans."

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Passengers who had been booked to fly to Fiji before Tuesday, February 23 would be able to adjust or cancel their travel plans at no extra cost.

Fiji Meteorological Service The forecast track for tropical cyclone Winson.

Air NZ had full details of available option in the travel alert section on its website.

Flight Centre New Zealand general manager Sue Matson said anyone booked to travel to Nadi on an Air New Zealand flight should contact their travel agent, or speak directly to the airline.

The cyclone, which hit Tonga last week, was detected eight days ago and tracked southeast before turning northeast and heading west towards Fiji.

Winds close to the eye of the cyclone were estimated at 185kmh and strengthening.

Fijian and Tongan authorities are warning people to prepare as the storm system's track has been unpredictable.

On Friday, Winston was north northwest of Vava'u, in Tonga, moving slowly towards Ronga and west towards southern Fiji over the weekend.

Tongan authorities issued alert warnings, which means the island group could be hit twice in one week by the same cyclone system.

The cyclone has picked up speed and the latest forecasts say Winston is expected to affect parts of the central Fijian island group, Kadavu and Suva.

The category four system is expected to cause flash flooding and damaging winds in some parts and could develop into a category five tropical system.

A forecast on Friday by the Fiji Meteorological Service included warnings of gale force winds, heavy swells, possible flooding and gusts of up to 250kmh.

'"Damaging or destructive winds may begin several hours before the cyclone centre passes overhead or nearby," the forecast said.

Fiji National Disaster Management permanent secretary Meleti Bainimarama said the public should monitor forecasts and heed precautionary warnings.

"Fijians must secure their homes, their properties, their businesses or their livestock.

"Fijians living in flood prone areas should also prepare for evacuation in case of flooding or damage to properties."

The MetService expects Winston to track in a southerly direction, with most predicted tracks east of New Zealand

"With tropical cyclones there is always some uncertainly associated with the tracks they will take, so we advise people in New Zealand and planning travel to the Pacific to keep up to date with the latest forecasts," the MetService said.