Sticky, humid conditions are set to continue in much of the country as a band of warm, wet weather originating in the tropics hovers over the country.

Much of the North Island and parts of the South Island have been experiencing 99 per cent humidity with isolated pockets of heavy - sometimes torrential - rain.

And while the rain may ease, the humidity will stick around a while longer.

"It is feeling more like the Cook Islands than New Zealand at the moment. It is very humid in much of the North Island," Metservice lead meteorologist Mark Todd said.

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Auckland and Christchurch have woken to 99 per cent humidity, while in Wellington it's 98 per cent.

A heavy rain watch remains in place for Waikato, east of Morrinsville.

Northland and Auckland's sodden conditions have shifted to the southwest overnight, with Tauranga drenched in 59mm of rain in the past 12 hours.

Marlborough, which had been pelted by 198.5mm of rain in the past 24 hours in its Western Ranges, is beginning to come through the worst of it.

In Nelson, the Nelson Tasman Emergency Management Group reported rivers had risen rapidly in the district causing localised surface flooding and slips.

But the worst may still be to come with Tropical Cyclone Gita upgraded to category 4 overnight and weather modelling showing it could find its way here.

Tropical Cyclone Gita has been upgraded to a category 4 cyclone overnight. Image / Weatherwatch NZ
Tropical Cyclone Gita has been upgraded to a category 4 cyclone overnight. Image / Weatherwatch NZ

The cyclone has already wrought significant damage in both Samoa and American Samoa, and is heading towards Tonga and then southern Fiji.

"It strengthened overnight, meaning the winds in the centre could get up to 167km/h," Todd said.

"It bypassed Niue overnight, fortunately the winds did too with just strong to gale winds there.

"It is now moving west towards the southern islands in Tonga, and has the potential to strengthen to a category 5 with winds in the centre reaching 200km/h.

"It is likely to pass much of Tonga tonight, and is likely to pass over or near the capital, Nuku'alofa."

Niwa principal forecaster Chris Brandolino said the latest forecast from the Fiji Metservice strengthened Gita to the highest level, category 5.

The Red Cross in Tonga is advising the public to be prepared as Cyclone Gita approaches.

The Fiji Met Service said on Sunday it anticipated Gita to enter Fiji waters on Monday night, and pass south of Ono-i-lau on Tuesday night.

Todd said Gita was likely to move west/southwest until the end of the week.

"One outcome is that it will come to New Zealand, but we can't say that with much confidence as the system keeps changing its mind."

Auckland was left mopping up after being drenched by 55mm of rain in 14 hours over the weekend, sparking flooding, downing trees and forcing the cancellation of Sunday's Big Gay Out.

The sodden conditions saw dozens of weather-related emergency callouts across the region.

Health warnings were also issued at dozens of beaches around the city on Saturday, due to concerns about wastewater overflow and poor water quality.

Rain in Auckland, Coromandel and western Bay of Plenty eased overnight, with a heavy rain band now over eastern Bay of Plenty.

However, later today the rain may return to those areas and also parts of Waikato.

There have been pockets of isolated showers around the rest of the country, especially in the Tasman district.

"It is usually sheltered there, but a good moist northerly flow like this is perfect for it to get some heavy downpours," Todd said.

Nelson Tasman Civil Defence said the severe weather had caused rivers to rise rapidly throughout the district with localised surface flooding and slips.

"Areas of most impact and concern have been Riwaka (Tasman District near Motueka) and Maitai River (main river in Nelson City)."

Speed restrictions are in place on SH63 from Renwick to St Arnaud due to flooding, a slip is blocking the southbound lane near the Richardson St intersection on SH6, there is flooding at the Spring Creek SH1 junction to Renwick SH6 junction, and Jacksons Rd between Old Renwick Rd and SH62 is closed.

Later in the week a few fronts will start to hit the South Island, bringing rain to some areas.

Those same fronts should ease the humidity, Todd said.

Much of the North Island will be becoming fine from Wednesday through Friday.


Today's weather


Northland, Auckland, Waikato and Bay of Plenty

Drizzle and dry spells, morning heavy rain about the Bay Of Plenty then easing. Humid northerly winds continue. Patchy rain may return later in the day or evening.
Highs: 25C-28C

Western North Island (including Central North Island)

The odd morning shower clears then sunny areas increase, northerly winds.
Highs: 25C-29C

Eastern North Island

Morning rain eases then becoming patchy, some sun may break through this afternoon. Northerly winds.
Highs: 26C-29C

Wellington

Patchy showers possible this morning then sunny areas break through, breezy north to northwesterly winds.
Highs: 23C-24C

Marlborough and Nelson

Any morning showers clear otherwise mostly sunny with northwesterly winds. Winds tend more northerly about Nelson in the afternoon.
Highs: 25C-30C

Canterbury

Mostly sunny with light winds then afternoon northwesterlies, right on the coastal fringe a sea breeze may keep some coastal spots a bit cooler.
Highs: 26C-30C

West Coast

Any morning drizzle clears then cloud breaks to sunny spells, about Fiordland rain develops in the afternoon however. Light northwesterlies.
Highs: 22C-27C

Southland and Otago

Mostly sunny with some developing high cloud, a few showers move through late afternoon / evening for Southland. North to northwesterly winds for most although tending east to northeast about coastal Otago.
Highs: 23C-27C