Weather: 'Exceptional' heavy rain, gales, floods set to slam New Zealand

Heavy rain and gale force winds are set to slam New Zealand in the next few days after a week of calm conditions.

"A change is coming for many this week, as a cold front moves over the South Island and lower North Island," says MetService meteorologist Rob Kerr.

"Warm, moist northwesterlies ahead of the front [are] set to bring heavy rain and gale force winds to southern and central parts of the country during the next few days."

Severe weather warnings for rain accumulation have been put in place for parts of the South Island.

As much as 600mm is forecast to fall on Fiordland and West Coast ranges between Sunday and Tuesday, while a massive 500mm is possible into the headwaters of Otago and Canterbury rivers.

"Even for these areas this is an exceptional amount of rain and could lead to significant flooding and possible road closures," MetService warns.

"The heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous."

Northwest gales could become severe, starting in Southland and Fiordland on Monday evening, and then affecting all of Otago and Canterbury during Tuesday. A MetService severe weather watch has been put in place.

"This is both a significant event, and a big change from current conditions," Kerr warns.

"Updates to current severe weather warnings and watches [are] likely as the system approaches, and further severe weather warnings and/or watches possible as it moves northwards during the week."