The fine weather expected this week, with some cloudy periods and afternoon light breezes, is due to a front over the southern South Island weakening as a ridge of high pressure moves onto the country.

Temperature highs in the Bay of Plenty should reach into the low 20s over the next few days with minimal risk of severe weather, with rain expected by Saturday.

On Wednesday, the ridge of high pressure over the country is set to move northeast as a front approaches from the Tasman Sea.

Northwesterlies will strengthen over the South Island and southern North Island, and rain is likely about the west of the South Island.

There is low confidence that the rain will reach warning amounts over Fiordland, a MetService spokesperson said.

Strong northwesterlies are expected in many areas as fronts move onto the South Island from the Tasman Sea on Thursday.

"Heavy rain is likely about Southern Alps and the west of the South Island," said a MetService spokesperson.

"There is moderate confidence that warning amounts of rain will fall in Fiordland, the ranges of Westland and the headwaters of the Otago and Canterbury lakes and rivers.

"There is low confidence that a rain warning will also be needed for the Buller and Grey Districts and the coastal areas of the Westland District.

"For inland Canterbury, Marlborough, Wellington and Wairarapa, there is moderate confidence that the northwest winds will reach severe gale strength."

On Friday a northwest flow is expected to remain over the North Island.

Fronts are expected to move northwards over the South Island and a low may cross the Island later in the day. Strong northwesterlies are likely around the centre of the country and rain is expected in many places.

"There is moderate confidence that a heavy rain warning will remain for the ranges of Westland and the headwaters of the Otago and Canterbury lakes and rivers during the morning," said a MetService spokesperson.

"For Fiordland, the coastal areas of the Westland District, the Grey and Buller Districts, western Tasman District, the Tararua Range and Mount Taranaki/Taranaki Mounga there is low confidence that warning amounts of rain will fall.

"There is moderate confidence that the northwesterlies will reach warning strength about Marlborough, Wellington and Wairarapa, while for inland Canterbury there is low confidence that a wind warning will be needed."