As much as 600mm of rain is possible for Tasman, and up to 400mm for parts of Nelson.
MetService has issued Orange Heavy Rain Warnings for the region, with each warning expected to remain in place for around 40 hours.
Tasman District northwest of Motueka could see 400 to 500mm of rain from 9am today until 2am Tuesday, with peaks possibly reaching 600mm.
Peak rainfall rates in these areas may reach 20 to 30mm per hour, and thunderstorms are possible from Monday evening.
The chance of a Red Warning upgrade for this area is considered moderate.
Meanwhile, areas between Richmond and Bryant Ranges, including the Rai Valley, 300 to 400mm of rain is expected from noon today until 4am Tuesday, with peak rainfall rates of 15 to 25mm per hour.
The likelihood of an upgrade to a Red Warning for Nelson remains low.
MetService warns that streams and rivers may rise rapidly, leading to surface flooding, slips, and hazardous driving conditions.
Residents are advised to avoid low-lying areas and drive with care.
A Heavy Rain Watch for parts of Tasman District not covered by the warnings will also be in place from noon today.
