Heavy rain and thunderstorms are in store for parts of the country this week.

A frontal system from the Tasman Sea will bring rain to several regions, with particularly heavy rain in southern and western areas, MetService meteorologist Lisa Murray said.

MetService has severe weather warnings in place for Fiordland and Westland.

Fiordland and parts of Otago will be the first hit with rainfall of up to 10 to 15ml per hour and thunderstorms expected today, she said.

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"There's a lot of trampers in those areas at this time of the year who will need to be careful," Ms Murray said.

"Same goes for drivers. There may be hazardous conditions so drivers need to take extra care."

Fiordland is expected to get between 100 and 150mm of rain today and tomorrow.

Westland is expected to get between 100 and 120mm of rain on the coast overnight on Tuesday and on Wednesday. A further 300 to 400mm of rain is expected in the Westland ranges.

A few showers are expected over the next few days in areas such as Northland, Auckland and Bay of Plenty, while much of the country will experience rain on Wednesday and Thursday.

"People should keep up to date with our online severe weather warnings," Ms Murray said.

Meanwhile, WeatherWatch is predicting possible flooding laster in the week.

"[A] big low will pull down a narrow but significant stream of moisture-rich tropical air and will generate some very heavy downpours across Wednesday and Thursday," said WeatherWatch head forecaster Philip Duncan.

"With a blocking high still out to our east it also means these rain bands could be slow moving -- increasing the risks of possible flooding."

Beachgoers should be mindful of big seas, Mr Duncan said.

"[Tropical cyclone] Winston is not directly hitting New Zealand but swells of 2 to 3m will make beach conditions in the Far North and Northland more dangerous over the next few days.

"Swells from the north or north east are also expected to build around some parts of northern Auckland and eastern Coromandel Peninsula.

"The cyclone will be tracking into slightly more unfavourable conditions for it to grow and may even drop down another category, however, it is not expected to weaken from a cyclone for several days."

A westerly change should blow the last of the tropical downpours out east of the country on Friday and set up most of the country for a dry, settled weekend, Mr Duncan said.

Main centres outlook for the week:

Whangarei: Fine spells with few showers. Rain Wednesday and Thursday but still humid.

• Auckland: Showers with rain late on Wednesday and periods of rain on Thursday.

• Tauranga: Cloudy periods and fine spells with few showers. Periods of rain on Thursday.

• Hamilton: Fine spells with showers turning to rain late on Wednesday. Periods of rain Thursday.

• Wellington: Fine with some cloud. Periods of rain and strong northerlies on Thursday.

• Christchurch: Fine with high cloud. Some rain possible on Wednesday and periods of rain on Thursday.

• Dunedin: Cloudy periods. Scattered rain from Wednesday afternoon.

Source: MetService