Heavy rain caused a slip to block the road out of Deep Cove on Sunday afternoon, leaving 339 people trapped.

Part of Fiordland is cut off and hundreds of tourists are being airlifted out, after torrential rain - 217mm of which fell in six hours - brought down a slip, blocking Deep Cove.

Heavy rain caused a slip to block the road out of Deep Cove on Sunday afternoon, leaving 339 people trapped. By 7pm, when the showers stopped, more than 300mm of rain had fallen.

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JOHN BISSET/STUFF MetService has heavy rain and strong wind warnings in place for parts of the South Island and lower North Island as a front moves northwards up the country on Sunday. (File photo)

Real Journeys Corporate Communications Manager Tsehai Tiffin said three helicopters were put on standby to get people out as soon as the weather cleared.

By 5pm the weather turned in their favour with the first lot of people flown over the slip before heading back to Manapouri. The helicopter trips were ongoing.

READ MORE: Canterbury's aptly named Terrible Gully turned to river of rock by ex-Cyclone Gita

Tiffin said the cost of flying the tourists was not a concern. She said the road would be inspected on Monday morning.

Deep Cove Outdoor Education Trust member Bob Hughes said there was more than 250mm of rain from about 3am on Sunday.

A school group was able to exit Deep Cove just before the road was blocked. No-one had been able to drive out since.

Motorists were earlier warned of hazardous driving conditions as a cold front sweeps in from the south on Sunday. It would move north over the mainland onto the lower North Island by Monday morning.

Stewart Island was first in the firing line, recording sustained northwest gales of 110kmh by 11am, gusting to 145kmh. At 6pm, MetService announced Milford Sound had been drenched.

"Milford Sound has recorded some very big rainfall totals this afternoon, with 217mm being recorded over the past 6 hours, and a peak rainfall rate of 40.4mm/hr," it said in a tweet.

During the day, many parts were hammered by strong winds. In a 10pm tweet, MetService said Mt Hutt recorded 185kmh gusts, Le Bons Bay in Banks Peninsula had a 156kmh gust, Wellington's Kelburn recorded a 135kmh gust, which coastal Christchurch was hit by 100kmh gusts.

The powerful front comes almost a week after ex-Cyclone Gita barrelled into the country from the tropics, turning rivers to raging torrents, closing roads and bringing down slips.

Heavy rain warnings are in place for Fiordland, Westland, the headwaters of the Otago and Canterbury lakes and rivers area from Arthurs Pass southwards, Buller and the Tararua Range.

Up to 150mm of rain was expected around the divide near the headwaters of the Otago and Canterbury lakes and rivers.

MetService meteorologist Andy Best said northwest winds with sustained speeds of 95kmh were expected in the Cook Strait by 1am on Monday.