A warning has been issued for trampers, campers and boaties to leave the Marlborough Sounds today as cyclone Gita approaches with heavy rain, severe gales and waves up to 6m high expected.
Residents are being told to be expect gale force winds, heavy rain and a storm surge along the Nelson-Tasman coastline from tomorrow morning and into Wednesday.
The Marlborough District Council says the heavy rain could cause slips, and waterways to rise rapidly.
People in coastal areas needed to be prepared for the storm to coincide with high tide overnight on Tuesday, which could generate waves in excess of six metres, the council said.
Civil Defence suggests residents in low lying coastal areas should consider staying elsewhere tomorrow night.
See how the storm is expected to impact on NZ with an interactive map here:
The national forecaster earlier said heavy rain would cause slips, rapidly rising streams and rivers, and flooding. There would be severe gales with damaging gusts, and high tide hitting overnight on Tuesday could bring waves larger than 6m high.
MetService's latest severe weather warning said coastal areas from Raglan south as far as southern Wairarapa were most at risk, while in the South Island coastal inundation was expected to be worst for areas north of Buller and North Canterbury.
Between 150mm and 200mm of rain was expected to hit west of Motueka, in Marlborough, Westland and the Canterbury High Country, with peak intensities of 20mm/h to 30mm/h.
Lesser but still serious amounts were expected elsewhere in the Nelson and Buller areas, while about 100mm to 120mm were expected over 24 hours in Wellington.
Severe gales were expected in Taranaki, Taihape, Whanganui, Nelson and Buller, Marlborough, Westland and Canterbury High Country, and Canterbury north of Banks Peninsula.
Civil Defence officials are telling people to tie down outdoor furniture and be wary of falling trees in the affected regions.
A week ago, the cyclone passed directly over Tonga's main island Tongatapu, flattening and flooding homes, sacking the parliamentary House, taking the roof off the forecasting office and severely damaging a church.
Samoa and some Fiji islands were also badly affected, with communications shut down and homes destroyed.
Earlier, Niwa forecaster Chris Brandolino told Morning Report the big impacts would come from wind, rainfall and waves.
The strongest winds were at the centre at the storm which was forecast to hit the top of the south including Farewell Spit, Buller, Motueka and Nelson, beginning on Tuesday but hitting a crescendo early Wednesday morning.
There would also be strong winds and rain stretching across the Strait to Wellington and up to Taranaki, Mr Brandolino said.
Wellington was likely to get a dowsing of heavy rain overnight into the early hours which could have an impact on commuters, he said.
"We could be talking potentially poor-drainage flooding and people trying to get into work when there's heavy rainfall.
He said ferry sailings could be affected with much of the heavy weather striking right around the Cook Strait.
Wellington's civil defence said commuters should seriously consider whether to stay home tomorrow, and workplaces should be discussing this with employees.
To the south there would be significant rainfall in Westport, Kaikōura and Christchurch.
Prime Minister Jacinda Ardern said the government was making sure there was a co-ordinated response to any damage as a result of Cyclone Gita.
The government was tracking Gita's progress and trying to get a sense of its magnitude, she said.
"My strong advice would be locally, keep an eye out for the advisories coming out from Civil Defence and MetService, and please take those advisories seriously and prepare as much as you are able."
Westland Civil Defence Officer Andy Thompson told Jessie Mulligan that Westland District Council had placed its emergency operations centre on standby in preparation for Gita, and called in emergency services and support agencies.
"By that I mean, we have the DHB, the health here, we have all of the roading agencies that will keep the roads open, ElectroNet who will do all the power, the welfare, a really big thing because if we do lose power which is more than possible, we need to local after the elderly in the old folks homes, we need to make sure that vulnerable people are particularly looked after in these situations."
A lot of preparation is being done including, putting generators in place and getting sandbags to those who need them.
The biggest issue is the south-easterly winds that have been forecast which come through the valleys and can be very damaging, Mr Thompson said.
"A lot of farm shed buildings, cow sheds those sorts of things are open to the east because that's not a prevailing wind ... if that wind does come in that direction with the force that is expected there could be a lot of structural damage."
West Coast leaders are meeting at 5pm to review the latest information about Cyclone Gita, and decide whether to declare a state of emergency.
Buller Mayor Garry Howard said the mayors and the regional council would be looking at MetService updates and what they meant for each district.
"We've got to wait and actually watch this weather pattern that is coming and make sure that we're doing it correctly, but there is an advantage of declaring early and getting in place everything that we need to do and having the authority to do that."
MetService is forecasting wind gusts of up to 150km/h for Westland and the Canterbury High Country.
Civil Defence director Sarah Stuart Black said it was unclear at this stage how likely it would be for a state of emergency to be declared.
"I think what we know is that if the forecast high winds, high rainfall that could create more storm surges could affect already vulnerable parts of the coastline or areas that could already have been affected.
She said there was a real need to be prepared for how the storm might play out.
"So that's about having a discussion within your home whether that's family of flatmates, whatever your home situation - about what you'll do if you do need ot evacuate or if the power goes out or the water is not on.
"And making sure that you've got a grab bag if you need to go quickly, remembering to take your pets with you."
She said they would be looking to Niwa and the MetService to provide information about where the storm might hit.
"At this point it's about making sure that their outdoor furniture, and rubbish bins and trampolines have all been secured, and then during the storm keeping doors and windows closed with the curtains closed as well - which will help protect if any glass breaks - but also being prepared to move if the situation changes."
She said civil defence emergency groups across the country had been preparing since Thursday, and would announce evacuation centres if and when they were needed.
The West Coast Emergency Management Group said it would be a significant storm with road closures and power cuts likely.
Locals are urged to store water supplies and check their getaway kits in case they need to leave in a hurry.
Residents should keep doors closed, secure items that may blow around and unplug small appliances in case of power surges, it said. They are also advised to bring pets inside and keep informed about weather and civil defence updates.
MetService said the ex-tropical cyclone was being preceded by a warm front today, while cold was moving up the South Island to reach the centre of the country by midnight tonight.
The heat means most of the North Island, Picton, Blenheim and much of Canterbury is set for temperatures around 20°C today, despite showers nearly everywhere except Northland and Hawke's Bay.
Heavy rain in Fiordland had also eased.