MetService warns the South Island will be hit with a "significant rain event" this week - and a "rare red warning" could be issued in the coming days.
A low-pressure system is transporting warm tropical air towards the western South Island, bringing heavy and prolonged rain.
An orange heavy rain warning is in place for Westland and Buller from Wednesday, while Fiordland, western Tasman, and the Canterbury and Otago headwaters have been issued a heavy rain watch.
"The current orange heavy rain warning stipulates that up to 550 millimetres of rain is expected in the South Island ranges," MetService warns in a release on Monday.
And there is the potential that the Westland orange warning could be upgraded to a red warning in the coming days.
MetService says its meteorologists will be monitoring the situation closely along with regional councils and hydrologists.
"Heavy rain could cause rivers to rise rapidly, localised surface flooding and slips which when combined with a King Tide could cause hazardous driving conditions," MetService meteorologist Ashlee Parkes says.
"People are strongly advised to keep up to date with the latest warnings and watches."
Although the rain will bring relief to dry parts of the South Island, regions in the northern half of the North Island will see little respite from the dry conditions with MetService forecasting just one or two light showers may sprinkle the northeastern coast.
"As this low-pressure system tracks towards the South Island, warm northeasterly winds strengthen across Aotearoa. Many will also notice the humidity rise over the coming days as this system drags muggy, tropical air across the country," Parkes says.
"We have some heat alerts in place across the country during the first half of this week as many regions see their maximum daily temperature push into the late 20s and early 30s, with little relief overnight."
