Auckland is looking to be the pick of the bunch reaching 26C both weekend days, and most places in the North Island are expected to reach the mid-20s.
Today it is expected to reach 24C in Auckland, with potential showers through the day.
A king tide on the east coast is due at 8.30am and the NZ Transport Agency warns it could bring some flooding on Tamaki Drive.
Auckland Transport says with early morning traffic and possible lane closures people should expect delays or consider alternative routes.
The weather today and tomorrow will be very similar, Dyason said.
"There will be a few showers in Northland, Auckland, and Bay of Plenty through to Gisborne.
"In the lower North Island there will be a lot of cloud, but they will be quite unlucky to get any showers with it."
The West Coast will feel the brunt of the weather.
A severe weather warning is in place for Westland south of Otira, with 200 to 300mm of rain expected to fall through to 1am Sunday.
"But the majority of that should fall in the ranges," Dyason said.
"Away from there will be periods of rain today and tomorrow in Nelson and down the centre to Fiordland.
"In the eastern parts will be a bit of scattered rain in Southland and Otago, and a few spots in Canterbury and Marlborough."
The unsettled weather continues a trend from February, which was far warmer and wetter than average.
The month broke February rainfall records at the top of the South Island, with Nelson Airport recording 234mm - 370 per cent of the normal rainfall - and 199mm falling at Blenheim Airport, four times the normal amount for the area.
Auckland recorded 149mm, more than two times its average February rainfall, and Christchurch's 121mm was nearly three times above normal.
Hamilton, Wellington and Dunedin all experienced more than 200 per cent of their average February rainfall too, recording 203mm, 171mm and 135mm respectively.
MetService meteorologist Georgina Griffiths said the amount of rainfall was unusual, but February was often "boom or bust" in terms of wetness.
"The month is often characterised by deluge rainfall – or none at all."
The extremes extended to the temperatures, which were also higher than normal in most spots. These scorching temperatures contributed to what is the hottest summer on record.
Climate scientist Jim Salinger said final figures for the month revealed the average temperature over the country was 0.9C above average.
"This puts the summer 2017/18 [temperature] at 18.8C, 2.2C above average," he said.
Temperatures recorded by Metservice averaged at 21.2C for Auckland, close to 1C hotter than the average temperature normally recorded for February.
Tauranga's average temperature of 21C was 1.4C higher than usual and Wellington had a 1.3C boost, to 18.5C. Temperatures down south were also slightly higher than normal.
This Sunday and even Monday show much of the same warm and humid weather across the country, with a potential change not arriving until Tuesday.
"From around Tuesday a front will start to arrive from the southwest, and will flush out a lot of this warm and humid weather," Dyason said.
"The South Island will see a slight dip in temperature then, but not too significant, just a change of pattern.
"It is still early autumn, and we could still get more of these warm, humid northeasterlies anytime."
Blocking systems like that keeping the humid system over the country are not uncommon, Dyason said.
"We get blocks to the southeast every now and then, which lead to an extended run of the same kind of weather.
"It all depends on where the high sits, though. At the start of spring last year the block was sitting over the country, which gave a long period of fine weather."
Today's weather
Whangarei Humid. Partly cloudy, with a few showers, chance heavy. Northeasterlies. 25C high, 20C overnight.
Auckland Humid and cloudy, with a few showers, chance heavy. Northeasterlies. 24C high, 19C overnight.
Hamilton Cloudy periods, and the odd shower. Northeasterlies. 23C high, 17C overnight.
Tauranga Cloudy, with a few showers. Northeasterlies. 23C high, 19C overnight.
New Plymouth Cloudy, with the odd shower. Northeasterlies. 23C high, 17C overnight.
Napier Thick high cloud. Northeasterlies. 24C high, 17C overnight.
Wellington Mostly cloudy. Light winds. 23C high, 17C overnight.
Nelson Occasional showers, less frequent in the evening. Light winds. 22C high, 17C overnight.
Christchurch High cloud, with a few spots of rain. Northeasterlies. 23C high, 16C overnight.
Dunedin Occasional showers, mainly from afternoon. Northeasterlies. 22C high, 16C overnight.
