Orange rain warnings are in force for the Westland and Tasman districts, while the rest of the coast is under a heavy rain watch.
MetService says up to 150 millimetres of rain could fall in the Westland ranges south of Otira today, which could cause surface flooding and slips.
In Tasman, 80 to 120mm of rain was expected in the ranges, and 50 to 80mm possible nearer the coast.
The warning for the Tasman district, west of Motueka, applies from 5pm. From late Tuesday evening 15 to 25mm peak rates were expected.
The rain was not unusual for the area but a little heavier than normal, MetService meteorologist Alec Holden told RNZ's Morning Report programme.
Heavy rain watches cover the ranges of Buller and Westland north of Otira, the headwaters of the Canterbury lakes and rivers south of Arthur's Pass and Fiordland, along with the Richmond and Bryant Ranges, the Rai Valley and northern parts of the Marlborough Sounds.
Holden said a front had come out of the Tasman which was "currently being squished by a pair of high pressure systems on either side of it".
This was causing a narrow and intense band of rain to be pushed directly up and over the Westland Ranges, he said, and people would need to be mindful around rivers and lakes.
