An active front is forecast to move onto the South Island from the west during Monday evening, preceded by heavy rain for western areas, and northwest gales, says a MetService spokesperson.

"There is moderate confidence of warning amounts of rain in Fiordland and the ranges of the Westland District, also for the headwaters of Otago.

"There is high confidence of northwest gales becoming severe about exposed parts of the Canterbury High Country, moderate confidence for Otago and Southland, and low confidence for the Canterbury coast and Plains and inland Marlborough."

On Tuesday, September 3, the active front is expected to move east across the South Island in the morning then the North Island in the afternoon and evening, preceded by a period of heavy rain for western and northern regions and strong to gale northwesterlies.

There is moderate confidence of warning amounts of rain for Fiordland, the ranges of the Westland District, and the headwaters of Otago and Canterbury south of about Arthur's Pass.

There is low confidence of warning amounts of rain in the Canterbury headwaters north of Arthurs Pass, also the Grey and Buller Districts, and northwest Tasman.

There is high confidence of northwest gales becoming severe about exposed parts of the Canterbury High Country early morning, and moderate confidence for most of Otago and Wellington, then low confidence for the Canterbury coast and Plains and Marlborough.

"A showery west to southwest flow is forecast across New Zealand [on Wednesday], with no severe weather expected," says a MetService spokesperson.

On Thursday showery west to southwesterlies are forecast to continue across the North Island, while a front is forecast to move onto the South Island with some rain in the west. No severe weather is expected.