A severe weather warning has been issued for the Coromandel Peninsula, northern Gisborne, Bay of Plenty and Taranaki.
MetService warned people living in those areas that rivers may rise rapidly and surface flooding was possible in vulnerable areas.
The weather was being caused by a humid northeast flow covering the North Island.
A front lying over the upper North Island is expected to move slowly south-eastwards today, preceded by a broad area of rain.
The heaviest falls were expected about the Coromandel Peninsula, Bay of Plenty, and the northern parts of Gisborne, where 80-120mm is expected to accumulate, also Mt Taranaki where 50-80mm is expected over a short period this afternoon.
Localised heavy downpours were possible in thunderstorms.
MetService duty forecaster Leigh Matheson said the weather was being caused by a broad, complicated and humid low currently moving across much of the North Island.
The low had come across the Tasman sea with a region of slow-moving northeasterlies ahead of it, she said.
"That allowed subtropical air to be pulled down over the North Island.
"That broad trough which is sitting out to the west is expected to move slowly east over the country."
The trough was mostly affecting the North Island, however Nelson was also affected, Ms Matheson said.
"There's a lot of low cloud around the coast of Canterbury, but they're generally not being affected by it."
The rain would move away from Auckland today, although there would still be a chance of heavy, thundery showers from the afternoon and into the evening, she said.
"Although some fine breaks are on the cards, there are still localised heavy falls to be expected."
The humidity would remain overnight and Aucklanders will wake to low cloud and patchy drizzle in the western areas tomorrow, to be followed mostly by fine breaks later in the day, Ms Matheson said.
When the trough moved away on Friday, is was set to be replaced by another moving down from the Tasman Sea, which would bring with strong norwesterlies and heavy falls which would affect the lower North Island and the West Coast in the South Island, she said.
- By Brendan Manning of APNZ
