Snow warnings are in place across much of the South Island as a cold blast sweeps up the country.

MetService says a cold south to southeast flow is expected to spread north over the South Island today, bringing snow and ushering in low temperatures for the next few days.

Heavy snow warnings and watches are in place for Southland, Otago and Canterbury today, while some areas can also expect heavy rain and severe northwest gales.

The forecaster says snow could affect several southern highways, including Dunedin's Northern Motorway and the alpine passes, throughout Sunday.

By mid-morning all highways were still open but there were some restrictions in place: on State Highways 85 (Palmerston to Kyeburn) and 87 (Outram to Kyeburn) chains are essential and the roads are closed to towing vehicles. Chains must also be carried by those travelling over the Crown Range.

There are heavy snow warnings for inland Canterbury, and heavy snow watches for Otago and Southland, where snow amounts could approach warning criteria above 200 metres.

MetService said the widespread snow was "likely to significantly affect travel through these areas".

Southeasterlies were likely to persist through to Wednesday, keeping temperatures low in the South.

MetService said temperatures were forecast to drop as low as -3°C and the highs were not expected to reach double digits until the end of the week.