A complex trough of low pressure is moving over the North Island on Monday, bringing rain or showers, says a MetService spokesperson.

This will be followed by a narrow ridge then a fast moving front which is set to move over much of the South Island.

Night time temperatures are expected to drop below 10 degrees Celsius this week in the Bay of Plenty.

Much colder air follows the front over the South Island, bringing snow showers to low levels in the south.

"There is low confidence of rainfall accumulations meeting warning criteria over Mount Taranaki and the Tararua Range," says a MetService spokesperson.

"Overnight and before dawn, there is also low confidence of significant snowfall to about 500 metres over Canterbury High country south of the Rangitata River."

On Tuesday, a disturbed, strong and cold southwest flow is forecast to spread over the entire country, then ease later in the day as a ridge moves onto the South Island.

"Snow to low levels over the South Island at first, with low confidence of warning amounts of snow above 300 metres from Southland through south Marlborough.

"For the North Island, there is low confidence of warning amounts of snow above 700 metres for the Taihape region."

On Wednesday the ridge is expected to move over the North Island, while northwesterlies become established over the South Island, bringing rain to the west of the South Island.

"There is minimal risk of severe weather [on Wednesday].

"On Thursday, the west to northwest flow strengthens ahead of an approaching front, bringing rain to many western areas and strong winds," says a MetService spokesperson.

"There is low confidence that rainfall accumulations could meet warning criteria over Fiordland and Westland District."