UPDATED 12.20pm: Snow showers are already falling in Southland, as the South Island prepares for another Arctic blast.

A severe thunderstorm watch has also been issued for the upper North Island, from Waikato through to Northland.

Northern parts of Waikato and southern Northland are expected to be worst hit by heavy rain, hail and strong winds.

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MetService meteorologist Paul Mallinson says rainfall could reach up to 30 millimetres an hour.

"If you get that for more than one hour in a location then that can mean surface flooding problems. The hail looks like it could exceed 20 millimetres."

The worst of the weather is expected to hit this afternoon.

Drivers are being urged to watch out as snow and ice make some roads precarious. 

The Catlins, Fiordland and parts of Southland are being dusted with snow - with the wintry weather making the Milford Road precarious.

MetService is predicting snow could fall lower than 200 metres in northern Canterbury, starting late this afternoon.

Meteorologist Lee Matheson reports the snow will get heavier today and warns alpine passes will be slippery.

"During the day it should get more into parts of inland Southland ... Dunedin should see some snow later in the day. As we get into tonight, that's expected to push north," she said.

"Milford Road has got a lot of snow, there is half a metre from yesterday. Lindis Pass could have a little bit of snow. Places to be aware of: the roads around Dunedin, especially State Highway One, Arthur's Pass is now starting to clear."

Blue Skies weather commentator Tony Trewinnard warns people need to be extremely careful.

"Higher levels in Canterbury and some sleet lower down. We may well see a few snow flakes mixed in with the rain showers later on in the day tomorrow, parts getting some snow today, it will be icy from tomorrow night onwards."