A cold snap is set to usher in sub-zero temperatures and a bleak 48 hours across New Zealand with school holidays starting on a wet and icy note.

A tropically-fuelled low packed with rain is starting to unleash on the top half of the North Island while flood-ravaged parts of the South Island are set for another round of intense downpours coupled with a polar blast that will bring widespread snow.

MetService is warning of a stark and sharp change in store for today with temperatures in the south taking a dive, coupled with wet weather, as a cold front sweeps up the island.

Heavy rain warnings and watches were this morning extended to include northern regions including Auckland, Northland, Coromandel Peninsula and Bay of Plenty. Heavy rain is forecast to fall for 24 hours from 3am Saturday to 3am Sunday.

"At this stage it looks like the heaviest rain will affect Auckland and Coromandel Peninsula and it is likely that a warning for those areas will be issued closer to the event," said the forecaster.

The alerts were previously issued for western regions of the South Island and the bottom half of the North Island through to Mt Taranaki. Up to 180mm is expected to fall in Kahurangi National Park at the top of the South Island.

MetService is warning rain forecast for the Tararua Range and Mt Taranaki may reach warning criteria when it starts falling after lunch today.

Those in affected central regions are being warned to watch for rapidly rising streams and rivers and the potential slips and hazardous driving conditions.

The mercury is set to plunge across the South Island to freezing by the end of today with southerly winds bringing rain and frigid air direct from Antarctica - starting in Southland in the morning and reaching Canterbury after lunch.

MetService says Gore and Invercargill were forecast to reach their maximum of 13C by 1am, while Christchurch was expected to drop from a balmy 23C in early afternoon to a wintry 10C by evening.

Snow is forecast to lower to 400m in parts of Canterbury, Otago, and Southland.

The forecaster is also advising road snowfall warnings are likely to be issued for South Island alpine passes.

MetService says by Saturday a low pressure system affecting the North Island would make for a rainy start to the school holidays while the South Island would have more settled weather.