Parts of the South Island and the lower North Island are set to be battered by heavy rain today and light snow is possible at Porters Pass and on the Milford Rd.

MetService says a trough is crossing the South Island bringing heavy rain to western parts of the South Island and lower North Island.

The heaviest rain is expected in the ranges of Westland and the Tararua Ranges where a heavy rain warning was issued last night.

Bursts of heavy rain are in place for Fiordland early this morning, Buller throughout the day and the Kapiti Coast and Wellington from later this morning.

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Gusty winds are also due to hit the eastern parts of the South Island and central New Zealand with a chance of severe gales in Stewart Island, Southland, Clutha, Dunedin, Southern Lakes, Canterbury high country and Marlborough early today.

Later this morning, gale force winds are forecast for Wellington and Wairarapa.

At Porters Pass in Canterbury a period of snow was expected to occur last night and last until tomorrow morning. From 9pm Sunday to 9am Monday, 2cm to 5cm of snow may accumulate on the road near the summit, with lesser amounts down to 600 metres, MetService said.

Last night, up to 17 vehicles were stuck in snow on the Crown Range in Otago, police said.

Heavy snow also resulted in two campervans and a further 10 to 15 vehicles stuck on the Crown Range - Cardrona Valley Rd.

The council and roading contractors had been advised, and Police units with four-wheel-drive vehicles were on their way to help, police said in a statement.

Motorists were being advised to take extra care on roads in parts of the South Island tonight.

In Westland, a tree came down on Haast Pass Highway and another fell on Whataroa Highway at Harihari.

In Canterbury, police said a tree had reportedly fallen on power lines on Homebush Rd, Coalgate.

"All drivers are being urged to drive to the conditions and report any road blockages to police."