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 percent humidity with isolated pockets of heavy - sometimes torrential - rain.

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

Much of the North Island and even parts of the South Island are experiencing 99 percent humidity.

"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.

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

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 down here.

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

"It strengthened overnight, meaning the winds in the centre could get up to 167kmh," 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 200kmh.

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

"Next week it is likely to move west/southwest until the end of the week, but we can't say much more after that.

"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."

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."

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 breakthrough, 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 northwesterly.
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 the northeast about coastal Otago.
Highs: 23C-27C