Cyclone Gita is set to bring 12m waves in areas near its centre when it hits New Zealand, leading to significant coastal damage and flooding.

After much uncertainty about when and where the cyclone would make landfall, WeatherWatch said today it could hit the upper northwest corner of the South Island on Tuesday afternoon.

"Certainly central New Zealand (upper half of the South Island and lower half of the North Island) appears to be at the greatest risk based on the past few days of various models - but this could still shift further north or south," WeatherWatch said in a statement.

"Of course this isn't yet locked in and with the storm still at least three days away this could definitely still change.

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"It becomes a much more complicated low by the time it reaches New Zealand which is why there is still uncertainty."

WeatherWatch said computer models showed the cyclone, which is currently a category three, was likely to drop to a category two before hitting New Zealand.

MetService said earlier it was expected to have become an ex-tropical cyclone.

At this stage both islands were at risk of flooding and wind damage on Tuesday and Wednesday. The upper South Island could also get very heavy rain.

MetService described it as "highly impactful severe weather".

Damaging winds could extend for hundreds of kilometres from the centre of Gita.

Models by the US Government's Joint Typhoon Warning Centre showed gales were possible from as far south as Dunedin to as far north as Taranaki, or even Waitomo and Central Plateau.

WeatherWatch warned that travel delays and cancellations were likely, including for those travelling by road, air and crossing Cook Strait, early to mid next week if Gita retained its strength.

MetService urged people to be prepared and check emergency kits had enough food, water, batteries and cellphone chargers.

WeatherWatch said it would likely be Monday before the precise area of landfall was certain.

Cyclone Gita ravaged parts of Samoa, Tonga and southern Fiji this week and last weekend.

By the time the cyclone hit Tonga it was a category four and a state of emergency was declared. Aid and supplies have been sent to Tonga by the Defence Force.