The humid weather is the result of a sub-tropical flow over the North Island.
Much of that is set to change in the coming days as a southerly front whips up the South Island today before reaching the North Island tomorrow, bringing a wet middle of the week.
Most of the bad weather today is affecting the South Island, MetService meteorologist Karl Loots said.
The front is already battering the bottom of the South Island, dropping the temperatures to the low teens and bringing rain and bitterly cold winds.
"A series of cold fronts are bringing rain to southern parts of the island, developing in Canterbury and Westland, then in the late afternoon and evening reaching the upper South Island."
No severe weather warnings or watches are in place, Loots said.
"In the North Island the weather is looking much more simple, with fine weather in most places, and the odd shower from the afternoon and potential thunderstorms in the ranges around Gisborne and Hawke's Bay."
The warm and humid weather should continue for most until the southerly front reaches the North Island tomorrow.
The front will shift to the west and form a low pressure system, lingering next to the North Island and bringing rain tomorrow through Thursday.
It will shift northeast gradually and a southerly will take over on Friday bringing cooler showers for much of the country.
The South Island will clear up quickly tomorrow before a weaker front moves in.
A potential cyclone forming in the tropics north of New Zealand was still very uncertain, Loots said.
"It is still a tropical depression at the moment, and under the responsibility of the Fiji Metservice. They have said there is a moderate risk of it transitioning into a tropical cyclone over the next 24 to 48 hours.
"After that it could go anywhere from north of New Caledonia to the east of New Zealand. It is extremely uncertain at this stage."
They would know more about it over the next 12 hours, he said.
Today's weather
Whangarei Fine spells. A few afternoon or evening showers. Light winds. 27C high, 19C overnight.
Auckland Fine spells with isolated showers, some possibly heavy from afternoon. Southerly breezes. 27C high, 18C overnight.
Hamilton Morning cloud breaking to long fine spells, but chance afternoon shower. Southwest breezes developing afternoon. 27C high, 16C overnight.
Tauranga Mainly fine, but chance afternoon shower. Light winds. 27C high, 19C overnight.
New Plymouth Morning cloud breaking, then mainly fine. Westerly breezes. 23C high, 17C overnight.
Napier Partly cloudy. A few afternoon or evening showers, possibly heavy and thundery. Light winds. 26C high, 18C overnight.
Wellington Cloudy periods. A few showers developing this morning with a southerly change. Turning to rain at night as southerly strengthens. 22C high, 14C overnight.
Nelson Cloudy periods. Southeasterlies and a few showers developing around evening. 23C high, 15C overnight.
Christchurch Cloudy with a few showers. Rain and fresh southerlies developing late morning, easing in the evening. 18C high, 12C overnight.
Dunedin Occasional rain and fresh southerlies, clearing early afternoon and becoming fine. 17C high, 10C overnight.
