After weeks of mostly fine weather for much of the country, heavy rain is expected in the north and west of the both islands.
MetService has issued a severe weather warning for Westland and northwest Nelson, as a broad trough of low pressure moves east towards New Zealand from the Tasman Sea.
"This trough is preceded by moist northerlies, bringing rain to the north and west of both islands," forecaster Leigh Matheson said.
The heaviest falls are expected in the hills of Westland, with 120mm of rain is possible tomorrow. The ranges of northwest Nelson are also expected to receive about 80mm overnight tonight.
MetService advised rivers and streams may rise rapidly, and slips are possible.