Weather: South Island to swing from heavy rain to frost risk as unsettled start to week kicks in

The South Island will swing from heavy rain to the risk of frost this week as an unsettled start to the week kicks in.

MetService is forecasting a front to start moving northeast over the country on Monday and Tuesday, bringing heavy rain to western parts of the South Island.

"Heavy falls for Westland Ranges where a warning is in force. Heavy rain watches are also in place for Fiordland, the headwaters of the Otago and Canterbury lakes and rivers, and Buller as well," says MetService meteorologist Andy Best.

"We also have a strong wind watch in force for Canterbury High Country for northwesterlies ahead of the front."

After this front a ridge of high pressure moves in from the west, meaning a "distinct chill in the air" for many places overnight Tuesday and overnight Wednesday.

"This leads to the possibility of light frosts in inland basins of Otago and Southland Tuesday and Wednesday nights," MetService says.

"Alexandra and Invercargill drop to around 4C around dawn on Thursday."

As the ridge of high pressure becomes firmly established across the country from Friday, New Zealand is forecast to have another weekend of fine weather, with temperatures close to average for March.