Extremely heavy rain is forecast for areas of the South Island that have been hard hit by floods recently.
MetService has issued a warning that heavy rain is expected in Fiordland north of Doubtful Sound, Westland south of Otira, and around the Otago and Canterbury headwaters.
An active front is forecast to move north across the South Island today, bringing rain to most of the South Island.
Between 1pm and 6pm today, 90 to 120mm of rain is expected in Westland.
Rain could be slightly heavier around Canterbury and Otago headwaters and in Fiordland.
Rainfall intensities of 10 to 20mm per hour are expected in Fiordland, Southland and Clutha, with 10 to 15mm per hour in Dunedin and North Otago.
The heavy downpours could cause streams and rivers to rise rapidly. Surface flooding and slips are possible and driving conditions could be dangerous.
Thunderstorms and hail might add to the severe weather in these areas.
Northwest gales could approach severe gale force in exposed parts of Fiordland, Southland, Otago and Canterbury, MetService said.
Tomorrow and on Monday, there is a moderate chance of severe northwest gales in Marlborough, Wellington, and Wairarapa including the Tararua District.
By late Monday, there is a slight chance of gales in exposed parts of southern Hawke's Bay.
MetService has also signalled a slight possibility of significant heavy rain from southern Waikato to the Tararua Range tomorrow and on Monday.
"Late on Monday and Tuesday, a ridge is forecast to spread onto the country from the west, then gradually weaken on Wednesday as another front approaches the South Island from the southwest," MetService said.
"There is a high risk of thunderstorms associated with the fronts as they move through Westland and Buller this morning and afternoon."
Thunderstorms and heavy rain might reach northwest Nelson and Taranaki this evening and tonight.
Marine warnings are in place for the South Island due to the risk of gale force winds.
Late last month, heavy rain caused severe flooding that washed out parts of an old landfill near Fox Glacier and damaged Waiho Bridge, closing access between Franz Josef and Fox townships on State Highway 6.