A bout of "significant" rainfall will hit the West Coast of the South Island on Wednesday, with more than 500mm of rainfall expected in some areas.
MetService issued a severe weather warning for heavy rain in Westland for Wednesday and said there is still a risk of a red warning being issued.
Severe weather warnings and watches are also in place for much of the South Island including parts of Canterbury.
"While the wettest weather is set for the ranges of the West Coast, strong north-westerly winds will push some rain over into the east as well," MetService meteorologist John Law said.
"Inland parts of Canterbury can expect some heavy rainfall driven across into the headwaters of the eastern rivers."
An orange heavy rain warning has been issued for Buller, Westland, the headwaters of the Canterbury lakes and rivers about and south of Arthurs Pass, the headwaters of the Otago lakes and rivers and Fiordland north of Doubtful Sound.
Up to 550mm of rain is expected to accumulate in the ranges in Westland from 1am on Wednesday to 9am on Thursday, with lesser amounts expected on the coast.
West Coast Emergency Management, local councils and other agencies are actively monitoring the region's warning.
"With the rainfall amounts forecast, we could see some impact across the southern parts of the region in particular. It's important people know what may happen as the event unfolds and take the necessary precautions," Civil Defence Group Manager Claire Brown said.
Sandbags are available for residents to protect their properties from flooding.
Metservice has also issued orange strong wind warnings in the Canterbury High Country and Fiordland from Tuesday.
A heavy rain watch is in place for Tasman west of Motueka and Richmond and Bryant Range including the Rai Valley area. A strong wind watch has been issued for Southland west of Riversdale and Winton, the Queenstown Lakes District and Central Otago.
Looking to the North Island, the warm temperatures are set to stick around and reach into the 20s for much of the country by the middle of the week. MetService said the highest temperatures are expected in the east, with Napier possibly reaching 29 degrees on Wednesday.
MetService said while rain impacts most regions this week, the longer-term outlook is for primarily settled weather this weekend.
