MetService is warning of extreme weather, with three days of steady heavy rain forecast for the top of the South Island

Heavy Rain Warnings have been issued for Buller, Westland, Marlborough, Nelson, Tasman, and Mount Taranaki.

Extra emergency services are being deployed to the Buller District, where the Emergency Operations Centre has been activated and sandbagging has been taking place.

Heavy rain has moved onto the West Coast and Tasman overnight, with 35.5 millimetres of rain recorded in Franz Josef in the 12 hours up to 5am.

Meteorologist Dan Corrigan says it's close to what they'd consider a Red weather warning, and the situation is being monitored.

He's urging residents in impacted regions to stay up to date, because this is looking like a big event.