Up to 400 people including tourists are trapped after the road to Milford Sound has been destroyed by rain.

A State of Emergency has been declared for Fiordland and MetService has issued its first ever Red Severe Weather Warning signalling an extreme weather wet which could cause significant impact and disruption.

Southland mayor Gary Tong said he didn't know exactly how many vehicles were trapped on the stretch of road between Milford and the Cascade at this stage.

Authorities are aiming to fly over before dark and if there are people stuck - get them out. The road is likely to be closed until the end of the week.

Staff will be assessing the weather later this afternoon to see if they can evacuate those with medical conditions.

MetService is also urging people in Westland south of Hokitika and Fiordland north of George Sound to act now to protect their animals and property as more rain is expected to drench the region.

The state forecaster has issued its first ever Red Severe Weather Warning, which was introduced last year to signal the most extreme weather events where significant impact and disruption is expected.

The warning comes as a State of Emergency is declared in Fiordland and 350mm of rain in the past 24 hours at Milford Sound has triggered flooding and slips and left Milford road closed. It is unlikely to be reopened until the end of the week.

A further 300mm to 450mm of rain is forecast for in parts of Fiordland and Westland by Tuesday afternoon causing more disruption to the area.

MetServices has warned the heavy rain will cause dangerous river conditions, flooding and slips and could lead to other roads being blocked and communities becoming isolated.

MetService have issued their first Red Severe Weather Warning, as very heavy rain moves onto the South Island.\\

There has been 350mm of rain in the past 24 hours at Milford Sound, closing Milford road. MetService expect a further 300-450mm of rain in parts of Fiordland and Westland by Tuesday afternoon, which will cause dangerous river conditions, flooding and slips. Some roads may become impassable, and communities may become isolated.

Much of the rest of the South Island is under warning for heavy rain as well, with most of the rain leaving our shores on Tuesday afternoon or evening.

This is the first Red Warning MetService has issued since introducing this new level of warning reserved for the most extreme weather events where significant impact and disruption is expected.

MetService meteorologist and head of weather communication Lisa Murray said: "This Red Warning is issued after consultation with the West Coast Regional Council and the local Civil Defence after careful consideration of the impacts the area could see in this kind of weather event."

There has already been significant rainfall in Milford Sound with the weather gauge recording 350mm in 24 hours till 10am this morning (Monday), closing State Highway 94 (the Milford Road).