Wet weather across central New Zealand has cancelled a university graduation parade and closed roads.

Victoria University this morning was forced to abandon today's graduation parade through Wellington streets because of the rain.

A night of intense wet weather has closed the highway between Renwick and St Arnaud because of flooding and many other roads around the top of the South Island are submerged or affected by slips.

Late this morning MetService issued the first road snow warning for the South Island as a freezing polar blast nears.

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Several motorists have needed rescuing after their vehicles became swamped in rising waters.

The most dramatic rescue happened about 3am when a house bus became stranded in a flooded river near Hope in the Tasman district.

A man had to be saved from his swamped mobile home by Coastguard in a surf lifesaving boat.

Police said there was bad flooding on SH6 near Rai Valley and near Renwick, outside of Blenheim.

They warned motorists to take extra care in the hazardous driving conditions.

Earlier a ferry service across Wellington harbour was suddenly stopped by the worsening conditions and commuters were warned to wear gumboots when a train station flooded.

Victoria University posted on its Facebook page that even though the parade was cancelled, ceremonies would go ahead as planned at the Michael Fowler Centre.

The intense rain was expected to ease this afternoon but the country is bracing for a bitter cold snap in the next 24 hours.

The MetService has issued a range of alerts as heavy snow is due to fall as low as 400m across southern regions for the next two days.

Here are the current Warnings and Watches for wind and snow. Wrap up warm its going to be a cold couple of days. You...

Posted by MetService New Zealand on Wednesday, 17 May 2017

Snow is expected to blanket Stewart Island, Fiordland, Southland, Clutha, Dunedin and parts of Central Otago.

Motorists have been warned the Milford Rd will be affected by snow from later today. Over the next two days up to 15cm of snow was expected to fall on the high pass over the next 24 hours.

At the same time gales are expected to pound central and southern New Zealand.