The predicted path of Cyclone Gita, according to MetService.

West Coast residents still reeling from a devastating storm a fortnight ago could be among those worst hit when Cyclone Gita, which wreaked havoc in Tonga, makes landfall.

MetService has issued a severe weather watch, warning the cyclone - likely to cross the country late Tuesday or early Wednesday - could bring highly impactful severe weather" to central New Zealand.

More than 100 millimetres of rain could fall within 24 hours in regions including Canterbury, Marlborough, Nelson, parts of the West Coast, Wellington and Horowhenua.

IAIN McGREGOR/STUFF Paula Dobbs' home on Snodgrass Rd, north of Westport, was flooded when ex-tropical cyclone Fehi smashed into the West Coast earlier this month.

Gale-force winds, with gusts of up to 180kmh in the Southern Alps, and costal inundation are also being predicted, with Nelson and parts of the West Coast likely to be worst affected.

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* Tongan families carve out new reality post-Gita

Civil Defence and Emergency Management director Sarah Stuart-Black said Gita "has the potential to pack a punch and cause a lot of disruption".

She urged people to prepare for the possibility of power cuts, water outages and road closures that could "leave you or your loved ones stranded".

People should have a "grab bag" ready in case they had to evacuate.

"If you don't have a household emergency plan, now's the time to sit down with your family or flatmates and get it done," Stuart-Black said.

"Stay safe by staying out of harm's way. Try and run any important errands before the weather hits so you won't need to do any non-essential travel in treacherous conditions, and make sure you secure outdoor furniture well in advance."

Civil Defence and Emergency Management would monitor the situation and was ready to coordinate assistance if it was needed.

The storm comes less than three weeks after ex-tropical cyclone Fehi smashed into the West Coast, destroying homes, closing roads and - once again - triggering discussion about the vulnerability of coastal communities.

West Coast Regional Council chief executive Mike Meehan was meeting with the Buller, Grey and Westland district mayors on Sunday to discuss what precautions should be taken.

Measures could include sandbagging, road closures and, in the worst case scenario, evacuations, however Meehan said this depended on what updated weather modelling showed.

The one positive of the cyclone striking so soon after Fehi was "people are aware of what can potentially happen". However, depending on the severity of the storm, it would be a blow to communities "getting their lives back together".

WeatherWatch head forecaster Phillip Duncan said severe weather could be expected from lunchtime Tuesday, ahead of Gita's centre making landfall Tuesday evening in the northwest of the South Island.

"It will come in on the West Coast again [following ex-tropical cyclone Fehi], which is unusual that the South Island is ground zero for a tropical storm."

Sustained gale force winds of 60kmh to 100kmh were expected from Taranaki south to Hokitika, with gusts of up to 180kmh in the Southern Alps, which was going to "tear the storm apart" into two centres,

Duncan said parts of the South Island were likely to see 36 hours of heavy rain from Tuesday into the end of Wednesday, with more than 100mm expected in inland Canterbury.

He described the intensity of what was expected in the east and north of the island as "West Coast rainfall". Dunedin, on the cusp of the rain event, was still at risk of flooding, Duncan said.

"Christchurch could get some flooding. There could be flooding in Marlborough, Wellington, Nelson and the West Coast - and the West Coast will have the storm surges as well."

MetService meteorologist Rob Turr said from 6pm on Tuesday waves would rise rapidly from Taranaki south to Westport, with some areas seeing waves as high as 5 metres.

"Coastal inundation is going to be a significant part of this event," he said.

Nelson was an area of particular concern. Turr said the waves would die down in the early hours of Wednesday morning, however they would likely still be large around high tide, which came around 2am in places like Westport.

On Sunday, the Christchurch City Council urged people to be prepared. Contractors were checking the stormwater network, and residents should secure outdoor furniture and check their gutters were not clogged, the council said.

Between 50mm to 100mm of rain was possible in Christchurch from Tuesday into Wednesday. "If you live in a flood-prone area, take all the usual precautions," a spokeswoman said.

On Sunday morning, Gita was in open waters to the south of New Caledonia and moving southwest at 4 knots.

MetService said it was expected to move back towards the southeast later on Sunday or early Monday, and begin to track towards New Zealand.

Complicating factors, is another low east of Samoa. MetService said any new low in the region could deepen into a subtropical system.

Another region of interest is the Cook Islands, where there is potential for a subtropical low to deepen. The chance of a low in this region to become a tropical cyclone is low.

On Sunday morning, MetService meteorologist Rob Kerr said the main centre of the low was tracking towards Nelson, over the Southern Alps, and would hit overnight on Tuesday.

"It's a large system so it won't just affect that area and that track may change slightly as we get closer."

The country will begin to see heavy rain associated with a warm air mass affecting central New Zealand from Monday evening into Tuesday morning.

Despite losing its form, the ex-tropical cyclone won't lose its force, Kerr said, and will pack a similar punch to tropical cyclone Fehi, which hit in early February.

"It's another major event for New Zealand. We're talking significantly strong winds, gusts over 100kmh for many parts of the country."

Big swells were expected to roll in from the north, affecting exposed coastal areas of New Plymouth down to Westland and Nelson, Kerr said, and combined with tides could cause coastal inundation.

"We'll be looking at how the high tides are going to be timed with those as well.

"Because it's coming from the northwest, the western side of the country will be exposed."

The only saving grace is that the storm will be moving quickly and will be clear of the country by Wednesday evening, Kerr said.

People can expect warmer temperatures and high humidity as the tropical air mass moves over the country, combined with wind and rain.

Severe weather forecasters were expected to begin issuing localised watches from Sunday evening.

Civil Defence & Emergency Management storm tips:

- Secure, or move inside, anything that could cause damage in strong winds.

- Close windows and doors, close curtains to prevent injury from breaking windows.

- Stay inside and bring your pets inside. If you have to leave, take them with you.

- Listen to the radio and follow the instructions of emergency services.

- Avoid non-essential travel during severe weather, and never drive through floodwaters.