Heavy rain may cause surface flooding, slips and a rapid rise of water levels in streams and rivers in Northland, Auckland, Waikato and Bay of Plenty today, MetService warns.
A front is forecast to slide north across central and northern New Zealand, bringing a shift in the weather for many parts of the country.
Aucklanders woke up to steady rain early today, with surface flooding quickly appearing on some motorways.
Heavy rain is predicted to hit northern and central areas today with the possibility of heavy snow falling on parts of the Gisborne and Hawke's Bay ranges.
A heavy rain warning is in place for Northland, Auckland, Bay of Plenty, Waikato, Taupō, northern Hawke's Bay and northern Taranaki.
MetService meteorologist Micky Malivuk said it was due to the active front tracking across the country.
In Taranaki over the past 24 hours in the mountain area they had about 285mm of rain, which Malivuk said was "quite a lot".
"It's [the front] been slow moving since yesterday and it is eventually going to clear the North Island today."
However, first it would bring heavier falls to the central and northern North Island, Malivuk said.
The heavier rain should gradually ease to showers across the country tomorrow morning, with the exception of the west coast of the South Island where it would be dry.
The heavy rain comes as total water storage in Auckland's dams falls just below 60 per cent of capacity, well below the historic June average of 84 per cent.
Yesterday Watercare spokeswoman Maxine Clayton told the Herald between 50-80mm of rain is likely to fall on Thursday in the Hūnua and Waitākere ranges dam catchment areas.
"This is the only significant rainfall forecast for the whole month of July," she said.
"Typically we see somewhere in range of 200mm during July, so potentially we're looking for a rain deficit of 250-375mm since January."
The January-June period was the driest on record for much of Auckland, Northland and the Waikato, according to the MetService.
July was expected to be wet but no more than normal for this time of year - and not enough to make up the deficit.
Auckland's residents were asked earlier this week to do their bit to save water and stave off the potential for water restrictions later in the year.
The front, followed by a cold southerly change, moves off to the east on Friday.
According to WeatherWatch, a cool down is coming but the warmer than normal sub-tropical air flow still covers the North Island today.
"It's quite a basic set-up - an enormous high that has influenced New Zealand's weather for a week and half now lies east of us.
"The size of this high means the anti-clockwise winds going around it are being pushed right up to Fiji, then pulled back down again over NZ.
"This has been the process since late Sunday but peaked over Wednesday and this morning in northern NZ."
Main centre forecasts today
Rain with some heavy falls, easing towards evening. Southwest change around midday. High 19C, low 9C
Rain with some heavy falls, easing towards evening. Southwest change around midday. High 17C, 8C
Rain with some heavy falls, easing this evening. Southerlies developing in the morning. High 17C, 6C
Rain with some heavy falls, easing in the evening. Northerlies changing strong southwest in the evening. High 18C, 8C
Cloudy periods. A few showers developing towards evening. Southerlies becoming strong. High 15C, 7C
A fine morning, then a few showers from afternoon. Southerlies. High 12C, 5C
Cloudy periods, chance shower. Southerlies. High 11C, 6C