Heavy rain and strong winds are lashing most of the South Island, with at least one road closed because of the avalanche risk.

Overnight, 113 millilitres of rain fell at Milford Sound Airport.

MetService says the wet weather is just getting started as a storm makes its way up the South Island.

It has issued an orange warning and says an active front from the Tasman Sea is moving north east over the island today and tomorrow, bringing heavy rain to the western and central parts of the South Island, and severe gales to the east.

Some of that rain is likely to fall in the Canterbury High Country, and Environment Canterbury is warning that could "test the still vulnerable flood protection works in the upper reaches" of the Ashburton River, which burst its banks last year.

MetService meteorologist Kathryn Hodge says 3000 lightning strikes were recorded early this morning, mostly at the main divide.

The weather will ease for Southland this evening, but will continue until tomorrow afternoon in Canterbury, she said.

Waka Kotahi's Tresca Forrester told Morning Report rain is expected to be particularly severe in some mountain passes and along the West Coast.

Avalanche risk after heavy rain has forced NZTA to shut the road into Milford Sound at the start of the second week of school holidays.

Part of Milford Road was closed yesterday due to an avalanche risk, and will likely remain closed today.

"Everywhere else is open but we're just advising extreme caution if you are heading out into the high country across the mountain pass. The rain is going to be really heavy and intense, particularly through places like Arthur's Pass, Burkes Pass and down into the MacKenzie Country, Twizel, Tekapo, and across the West Coast as well."

People shouldn't underestimate the wind factor, she said.

"The wind is forecast to be very strong in places and particularly if you are heading away with caravans, high sided vehicles like a campervan, just really take extra care because it can sneak up on you on the road."

Severe northwesterlies are expected in exposed places in Canterbury, gusting up to 150km in the High Country.

Warnings of north west gales gusting up to around 120km are in place for Marlborough, Otago, Southland, Fiordland and Stewart Island.

"The advice is, things can change really quickly."

The wild weather has forced the closure of a popular tourist route, in the midst of the school holidays.

The transport agency has closed parts of State Highway 94 between Te Anau and Milford because of the risk of an avalanche.

Southern Discoveries chief executive Tim Hunter says every winter the business factors in 12 to 15 days where it can't operate - although it is usually later in the season.

"We've been watching this storm come for a few days with a bit of trepidation because we're talking about today, around 250mm of rain ... it's about a third of the annual rainfall for a city like Dunedin, in one day."

Business is about 30 percent up no last year, he says, with a lot of international visitors adding to the customer base.