Ex-cyclone Gita is gaining momentum as it moves towards New Zealand, and is likely to split in two as it moves nto a climate quite different from the one in which it formed.

The storm is barrelling towards New Zealand and is expected to make landfall this afternoon.

Most of central New Zealand, Westland and the Canterbury high country have been warned of flooding, winds up to 140km/h and coastal areas can expect waves up to 7m high.

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The weather is wreaking havoc on Kiwis' transport plans, closing SH1 north and south of Kaikoura and cancelling flights and trains.

MetService meteorologist Ciaran Doolin said the storm was gaining momentum as it moved towards New Zealand - despite the fact it had been downgraded to an ex-tropical cyclone.

"We re-classified it last night from a tropical cyclone. Now we are just calling it Gita," Doolan explained.

"This system, by the looks of things, is being supported by a trough in the upper altitude that is helping to develop it.

"It will eventually strike the east of the country."

The storm is also expected to split in two as it travelled southwards.

The cyclone would pass in the same general direction, but part of it would split off and float around the west of the South Island.

Doolin said the forces that drive a tropical cyclone were quite different from what drove a mid-latitude cyclone.

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"A tropical cyclone gets its force from warm sea temperatures when it forms in the tropics," he said.

"This started as a tropical cyclone then came down and turned into a mid-latitude cyclone."

MetService was warning the change in classification didn't mean the storm would have any less of an impact on the country when it did strike.

MetService had issued a heavy rain warning for North Otago, which can expect 60mm to 90mm of rain at up to 10 to 15mm an hour for the 24 hours from 1pm today.

Christchurch City Council says it expected flooding at low points and around low-lying river areas, and flooding where any blockages occur. It expects that streets around low-lying river areas will need to be closed later today.

Defence Force personnel had also been deployed to areas expected to be hardest hit by Gita.