Huge rocks, some weighing 200 tonnes, have crashed down on to Milford Rd in the South Island, forcing the only road in to Milford Sound to close.

Rocks the size of cars plunged down the hillside last night and have blocked Milford Rd, the only road access into the popular tourist destination.

Landslips and avalanches aren't uncommon in the area, but the size of the boulders that came tumbling down the hillside last night were significant, New Zealand Transport Agency Southland area manager Peter Robinson said.

"Some of the rocks are estimated to be 200 tonnes. They're three quarters of the width of the road," he said.

"Slips of this type are not uncommon. They do happen but in general the more common slips are much smaller."

The landslip stretches across 200 metres between Falls Creek and Monkey Creek. It occurred on the same site as the "Red Slip" which happened about 25 to 30 years ago, Robinson said.

There has been heavy rain in the area lately and NZTA decided to close the road yesterday afternoon due to an avalanche risk.

There were four avalanches and two smaller rock-slips on the road overnight, Robinson said.

Helicopters with geotech experts on board were canvassing the area today to determine whether there were any loose rocks perching at the top of the cliff, and the potential of further slips.

Once it has been determined that the area is stable to work in, equipment and staff will be brought in to start clearing the road. Some rocks will need to be broken up and it was too early to say when the road would be cleared, Robinson said.

Milford Sound Development Authority operations manager and Milford Community Trust trustee Andrew Welsh said residents and businesses were prepared for landslips and avalanches.

"Probably on an annual basis we allow for the road to be closed for two weeks a year," he said.

"It's not unusual. It's more of an inconvenience than anything."

People generally had two weeks worth of food, just in case, and flights in and out of Milford Sound allowed for people to get in or out if needed.

"We allow for it, we prepare for it."  

- © Fairfax NZ News