Aucklanders are being told to brace for rainfall of up to 35mm an hour, with a severe weather warning in place.
MetService has issued a number of severe weather warnings as a front carrying heavy rain sweeps over much of the country today.
The front is expected to move east over the North Island today, bring localised downpours and possible snow to some areas.
Warnings or watches are in place for the regions in the top half of the North Island - from Taranaki, Taupō and Napier northwards.
Auckland Civil Defence tweeted that intermittent bursts of heavy rain are expected in the region until late this afternoon, with some downpours reaching intensities of 25-35 millimetres per hour.
Heavy rain north of Auckland had, as of noon, caused surface flooding which had blocked the right-hand lane of State Highway 1 on the Northern Motorway, NZTA said.
Surface flooding and slips are possible, and heavy rain could cause streams and rivers to rise rapidly.
Motorists are reminded to lower their speed during rain and to maintain longer following distances, as rain can dramatically affect their ability to stop in an emergency.
Some snow is expected on the Napier-Taupō Road higher than 700 metres this evening and overnight, and some may settle on the road near the summit between 9pm and 3am.
Rain is also expected to turn into snow on the Desert Road above 800 metres tonight, with between four and eight centimetres of snow settling.
Auckland is currently experiencing a water shortage, with total water storage in Auckland's dams below 60 per cent of total capacity, and 30 per cent lower than what they usually are at this time of year.
However, the heavy rain today will not be sufficient to replenish the water supplies, with a longer period of sustained rain needed to totally refill them.