The Samoan government has issued a tropical cyclone warning as meteorologists upgrade the risk that Cyclone Gita will form in the Pacific.

The chance of a tropical cyclone forming over the weekend is now moderate to high, but forecasters say they can't predict where it will hit just yet.

Samoa has put out a Category 1 tropical cyclone warning for the next 12-24 hours, with wind speeds between 62-87km/h possible.

The Fiji Met Service has also upgraded the cyclone risk to moderate-high, with the next update due at 5pm.

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Niwa forecaster Ben Noll said west of Samoa there were "healthy areas of heavy rain and thunder" which had a moderate to high chance of becoming a cyclone in the next 24 to 48 hours.

WeatherWatch.co.nz said yesterday it was too early to lock in whether the probable cyclone would hit New Zealand - but warned if it formed it would be "much bigger than Fehi".

Noll said the weather system would probably strengthen over the weekend as it tracked to the east of Samoa and American Samoa.

"Over the weekend we will be watching to the east of Niue - it could strike as a powerful system."

Volunteers clean up the beach at Greymouth after ex-cyclone Fehi. A forecaster has warned the impending Cyclone Gita could be much bigger than the early February storm. Photo / Supplied
Volunteers clean up the beach at Greymouth after ex-cyclone Fehi. A forecaster has warned the impending Cyclone Gita could be much bigger than the early February storm. Photo / Supplied

Noll cautioned that until tropical cyclones actually form, computer models are poor at forecasting their strength or where they are headed.

"That said, in American Samoa, Samoa and later Niue and Tonga, that's the area it's likely to be in. Wind, rain and waves are the expected impacts but of course these islands are very small - if it passes just offshore or to the east the impact could be less."

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Noll said it would be "very premature" to predict where the system might track next week, but all Pacific nations should be keeping an eye on it.

Very warm ocean temperatures in the south Pacific would be ideal fuel for the cyclone to strengthen if it does form, he said.

"We have favourable conditions in the atmosphere so - track aside - it's a pretty good looking setup for tropic cyclone genesis. Perhaps the best we've seen."

MetService meteorologist Nick Zachar said the potential cyclone was "still looking a bit disorganised at the moment" and was not yet a named system. But he agreed the cyclone - which would tentatively be named Gita - would drift east, with Samoa, American Samoa and Niue in line for bad weather.

The islands could see heavy rain and flooding, he said.

"The models have it hooking southeast and then gradually south. Then [the models] are a bit more scattered until day 4-5 when it will gradually strengthen as it moves to the east."