MetService is forecasting a front that will be moving northwards over the South Island during Friday, August 23.

"Warnings and Watches extend into Friday for some areas," says a MetService spokesperson.

"There is high confidence that rainfall amounts will reach warning criteria in Westland, with significant spillover into the headwaters of the Otago and Canterbury lakes and rivers.

"Heavy rain is also likely in Fiordland, the Grey and Buller Districts and western Tasman and there is low confidence that a rain warning will be needed in Fiordland and moderate confidence that the other places will be covered by a rain warning."

Gale northwesterlies are possible in many eastern areas of the South Island and about the southern North Island and there is low confidence they will reach warning strength in Canterbury and Marlborough and moderate confidence that a warning will be needed in Wellington, including the Remutaka Hill road.

On Saturday the front is expected to move onto the North Island and weaken.

Meanwhile a ridge of high pressure should move onto the South Island.

Heavy rain over the South Island is expected to ease but there remains low confidence that rain warnings will extend into Saturday for western Tasman and the Buller, Grey and Westland Districts, as well as the headwaters of Canterbury and Otago.

For the Tararua Range and Taranaki Mounga/Mount Taranaki there is moderate confidence that a rainfall warning will be needed.

"Over a broader area of Taranaki, Waitomo, Taumarunui and about the Tongariro National Park there is low confidence of warning amounts of rain," says a MetService spokesperson.

"Late in the day on Saturday, there could be heavy rain about eastern Bay of Plenty and inland Tairawhiti/Gisborne with low confidence that a warning will be needed."

There remains moderate confidence that northwest winds will reach warning strength about Wellington, including the Remutaka Hill road.

On Sunday, August 25, the front is expected to become slow-moving over the North Island.

"The ridge of high pressure over the South Island should move out to the east late in the day.

"Areas of the North Island are likely to have rain, with low confidence that heavy rainfall warnings will be needed for eastern Bay of Plenty and inland Tairawhiti/Gisborne."

On Monday, August 26, a front moving east from the Tasman Sea is expected to bring a strengthening northwest flow over much of the country.

"There is low confidence that a heavy rain warning will be needed for northern Fiordland and the Westland District.

"There is also low confidence that northwest winds will reach severe gale about inland areas of the eastern South Island and about Wellington, including the Remutaka Hill road."