MetService is urging those in Golden Bay, Nelson, and the entire Northland region to prepare for potential flooding.
A trough embedded in a humid northerly air flow is expected to bring heavy rain on King's Birthday.
Orange heavy rain warnings are in place for Westland south of Hokitika, parts of the Tasman District , Ranges of Buller and Richmond and Bryant Ranges. Other parts of the South Island are also under heavy rain watches.
Meteorologist Paris Marshall said Golden Bay, in the Tasman district, had accumulated 278 millimetres of rain in the last 24 hours.
"Which is a lot of rain, concerningly a lot of rain, so they are very vulnerable to flooding, if not already."
While the heavy rain warning issued for Northland did not start until noon, Marshall said Kaitaia had close to 30 millimetres of rainfall in an hour, from 5am, "which is very persistent rain."
Mount Cook also had 147 millimetres over the last 24 hours.
Golden Bay, Nelson, and the entire Northland region would be affected most, Marshall said.
While the West Coast would also be pelted in the storm, Marshall said they were used to dealing with that higher level of rainfall.
Forecaster Brian Mercer said inland Nelson saw around 200 millimetres of rain on Sunday.
Marlborough's Emergency Management team has been preparing for heavy rain. The district is under an orange heavy rain watch for 22 hours that began at 6am Monday and lasts until 4am Tuesday.
The council on Sunday said that based on the current MetService forecast, high water levels are expected in the Pelorus River which is likely to cause flooding at Dalton's Bridge on State Highway 6.
It says there is the potential for prolonged rainfall on Tuesday morning if the weather system stalls over the southern Richmond Ranges.
Motorists are being asked not to travel overnight for the next few days, especially on State Highway 6 in the Havelock/Pelorus/Canvastown area because the road could be flooded or closed
Mercer said MetService will be keeping a close eye on the Tasman district overnight.
"They will also have the latest model runs and they'll be able to see how the system is evolving and how it continues to evolve for the next few days."
Taranaki Maunga and Northland also have orange heavy rain warnings in place, with possible severe gales in parts of the North Island.
Strong wind watches are in place for Taranaki, Auckland and Great Barrier Island.
