It's hard to imagine today as much of New Zealand basks in sunshine but snow and rain are on the way, with some farmers warned their newborn livestock could be at risk.

MetService has issued snowfall warnings for Milford Rd tomorrow while Tuesday will see more road snow, including on the Desert Rd. Westland and Buller are also under heavy rain warnings.

From 3pm today till Monday morning, 110-180mm of rain could accumulate about the ranges in Westland north of Bruce Bay and 70-100mm near the coast.

Peak intensities of 15-25mm per hour were expected about the ranges, MetService said.

That's enough to cause streams and rivers to rise quickly, with potential for surface flooding and slips.

Hail is also on the cards for the first half of the week. The cold snap would be a noticeable contrast to the warmer settled weather most places experienced this weekend, MetService meteorologist Stephen Glassey said.

"The whole country should experience changeable and unsettled conditions during the first half of this week, so everyone should keep up to date with the latest forecasts," Glassey said.

"It is not unusual to get weather like this during spring. Just last week we had a similar cold outbreak, and we will likely see some more cold outbreaks before spring is over."

Weatherwatch has also warned wind chill could bring temperatures down below zero in the south of New Zealand.

It will be a shock to the system after the heat that's spread over the country this weekend with daytime highs 4-8C above normal and some places expected to climb over 20C, Weatherwatch said.

Monday would see the cold air return, peaking on Tuesday and easing on Wednesday. That brought the potential for brief snow flurries to low levels in the south of New Zealand.

Weatherwatch said Tuesday's temperatures would be particularly miserable for stock, with sub-zero chills overnight and lingering cold on Wednesday.

The lower half of the South Island could expect single digit highs in some areas, Weatherwatch said.

Northern Southland and some parts of Otago could see snow flurries down to 100m or 200m. However it was the combination of cold and dampness that was more worrying for newborn livestock, the forecaster said.