People living in the hills east and north of the Bay of Islands are asked to be wary of rapidly rising streams. Photo: RNZ / Rebekah Parsons-King
An orange heavy rain warning has been issued for Northland, north of Whangārei, with as much as 250 millimetres of rain possible in some areas.
A slow-moving trough over Northland was expected to bring periods of heavy rain, possibly with downpours and thunderstorms, a MetService spokesperson said.
Up to 150mm of rainfall is forecast in 24 hours until 10am on Thursday, but it could peak at 250m in some places.
The worst of the rain is expected on the hills east and north of the Bay of Islands.
People in those areas are urged to clear their drains and gutters, be wary of rapidly rising streams, avoid low-lying areas and drive cautiously.
The rainfall rate is likely to peak around 15 to 25mm/h - anything over 6mm/h is classed as heavy rain - but rates as high as 40 to 50 mm/h are possible during localised downpours.
Rainfall of that intensity could cause surface or flash flooding, especially near streams or in narrow valleys, and could lead to slips.
Driving conditions could be hazardous due to surface flooding and poor visibility.
A severe thunderstorm watch had also been issued for the same area.