Civil Defence is warning Kiwis to be prepared as Cyclone Gita is expected to make landfall on Tuesday, bringing gale-force winds and heavy rain across several parts of the nation.

Strong swells, gale force winds, heavy rain and, in Canterbury, snow, is expected from Tuesday as the cyclone migrates to New Zealand after devastating Samoa and Tonga last week.

Ministry of Civil Defence and Emergency Management (MCDEM) director Sarah Stuart-Black said the Ministry would be monitoring the situation and was ready to co-ordinate assistance if it is needed.

"Cyclone Gita has the potential to pack a punch and cause a lot of disruption. Now is the perfect time to plan ahead.

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"This means preparing for the possibility of power cuts, water outages and road closures that could leave you or your loved ones stranded. It's also a good idea to have a grab bag ready in case you need to evacuate."

If you do not have an emergency plan then now is the time to get together with family or housemates to get it done.

MCDEM advised people to secure, or move inside, anything that could cause damage in strong winds, as well as closing windows and curtains to prevent injury from breaking windows.

People should stay inside and bring pets inside and if they have to leave, take them with.

Listen to the radio and follow the instructions of emergency services and avoiding non-essential travel during severe weather was essential, the Ministry said.

Cyclone Gita is honing in on New Zealand and is expected to hit the South Island's western coast hardest but forecasters said almost everyone would experience some wet or windy weather.

WeatherWatch forecaster Philip Duncan said the worst swells would be from New Plymouth to Hokitika, but Westport could be "clobbered" by the centre of Gita, with dangerous winds and coastal conditions.

Gita would approach the western coastline of the South Island on Tuesday night, bringing heavy rainfall and violent winds with wind gusts in wide areas in the northern South Island.

After Tuesday night heavy rainfall is expected to persist mainly in the central South Island, yet the distribution of strong wind and rainfall is uncertain, as models disagreed on the development of the cyclone.

The predicted ex-tropical cyclone would be preceded by a front that would hit the south of the North Island on Monday.

The activated front would produce heavy rainfall.

MetService meteorlogist Claire Flynn said the extent of the cyclone would be "widespread, crossing through central New Zealand".

Even places not on watch could get rain and wind, she said.

MetService had a heavy rain warning for Nelson, Buller District, Kapiti, Wellington, Marlborough, Canterbury, Westland and North Otago from early morning Tuesday.

Strong wind watches were in place for Taranaki, Taihape, Whanganui, Nelson, Buller, Westland, Wellington and Marlborough from Tuesday afternoon.

"Pretty much across the board everyone will see something," Flynn said.

People under the weather watches should be taking step to prepare for the worst.

"Things like bringing in outdoor furniture, cleaning gutters and checking moorings for boats, go to the GetThru website, they can give a breakdown in place.

There is also a possibility of unseasonable snow in Canterbury as the cyclone drags cool air with it.

Snow is predicted to fall in the Canterbury High Country on Tuesday.

It may affect some of the alpine roads around Canterbury, MetService said.

People in the watch areas should be taking step to prepare visiting the Ministry of Civil Defence and Emergency Management (MCED) website, getthru.govt.nz.