Tropical Cyclone Gita has strengthened to a severe category three storm and is set to further intensify as it makes its way across the Pacific.
WeatherWatch last night said the cyclone, which has already caused considerable damage and flooding in Samoa, was likely to become a category four in the next 24 hours.
And according to Niwa, a potential second tropical cyclone was developing to the west and northwest of Gita. The new cyclone would be called Hola.
Tonga is expected to be next in the firing line of Cyclone Gita.
WeatherWatch analyst Philip Duncan says the first cyclone is to reach Tonga tomorrow afternoon or evening.
He's expecting damaging winds with gusts over 200 kilometres an hour, waves over 10 metres and very heavy rain.
Philip Duncan says cyclones have a mind of their own so forecasters will need to wait another day or so before picking whether Cyclone Gita will impact New Zealand.
Meanwhile, New Zealand's summer continues to be put on hold — with the country in for yet another drenching as the super-soaker weather system shows no sign of easing.
A severe thunderstorm watch is in place for parts of the country today and MetService is warning people to be prepared for heavy rain.
Heavy rain could hit Northland, Auckland, Coromandel Peninsula, Waikato, Waitomo, Taumarunui, Taupo, Taranaki, Taihape, Whanganui, Manawatu, Tararua, Kapiti-Horowhenua, Nelson and Buller, MetService said.
"Rainfall of this intensity can cause flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips," MetService said.
"Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain."
The big wet hammering the top of the North Island is set to intensify today as it spreads south, bringing heavy rain and the risk of flooding, slips and thunderstorms.
But it is still too soon to say what impact Cyclone Gita might have on New Zealand later this week.
MetService has issued heavy rain warnings for Mt Taranaki, Nelson and western Marlborough, where up to 130mm of rain was expected to fall by midnight tonight. Up to 180mm was set to fall about the ranges west of Motueka.
MetService warned heavy rain in these areas may rapidly fill streams and rivers, cause surface flooding and slips and hamper driving.
Warnings have also been issued for Buller and Westland from this morning through to 3am Monday, where peak intensities could reach 25mm per hour.
It was a wet and muggy start to the weekend for the North Island, with humidity levels sitting at 100 per cent in parts of Northland yesterday. There was 72.6mm of rain recorded as falling in Mahia, in the Hawke's Bay region, in the 24 hours to 5pm yesterday.
Almost 53mm of rain fell in Kerikeri in the same 24-hour period. Auckland didn't escape unscathed, with a weather station in Warkworth recording 43mm of rain.
A complex low-pressure system with a series of warm fronts is set to bring more wet weather to the North Island today and tomorrow.
MetService has issued a severe weather watch for much of the island.
A heavy rain watch was last night in place for Northland, Auckland, Waikato, Waitomo, Taumarunui, northern Taranaki, western Taupo, Taihape, Horowhenua, Kapiti Coast, Tararua range and Wellington through to early Monday morning.
Gita was set to pass near Niue last night before heading just to the south of Tonga, where there was a chance of sustained wind speeds reaching more than 150km/h and waves over 10 metres.
In a statement WeatherWatch said Tonga could face coastal inundation, rain, flooding, storm surges and wind damage.
WeatherWatch said Gita was now much more powerful than recent Cyclone Fehi, which smashed a large swathe of the country last week, flooding homes and businesses, stranding travellers when damaged roads were closed and causing dramatic storm surges.
The north and west of the South Island were worst affected.
Sustained winds over 150km/h and gusts over 200km/h were forecast for mid next week by which time Gita was expected to sit out at sea south of Fiji.
The cyclone's forecast track turns clockwise before it starts moving westward rapidly next week.
This would take it north of New Zealand and close enough to seriously monitor, WeatherWatch said.
WeatherWatch said multiple reliable models suggested that Gita would track further southward towards the New Zealand area around next weekend or early the following week.
"It's still far too early to know if it will directly impact New Zealand - it remains one to watch and WeatherWatch.co.nz hopes to have more specific details this Monday about whether or not this will be something for New Zealanders to seriously focus on," WeatherWatch said in its statement.
"As of Saturday night it was looking increasingly likely that it would come into the New Zealand area around next Sunday or Monday. This may still change - it's not locked in."
According to Radio New Zealand Pacific, a state of disaster has been declared in Samoa after Gita caused high winds, widespread flooding and power outages on Friday night. There were also reports of uprooted trees and roofs being torn off in the Pacific nation.
RNZ reported the Samoa disaster management office saying more than 200 people were in evacuation centres around the country.
The Samoa Observer said there had been no reports of casualties.