One person is dead after a car was found submerged in a culvert in Southland about 10am.

A witness at the scene said the saloon car was not visible from the road, but had been spotted by a truck driver - who could see due to the height of his own vehicle - on State Highway 94 near Balfour this morning.

"A normal car driving up the road couldn't see it," the witness said.

"The car was two thirds submerged under the water. He called me and I went and tried to have a look, while he called 111."

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He was unable to see the driver of the vehicle until it was moved by emergency services.

The man said he believed the driver was a dairy worker, who had reportedly not turned up to work.

"He wouldn't be very old."

He said the car may have been there for some time.

The witness said there were tyre marks on the road, and he believed the vehicle had gone into a ditch, across a driveway, end over end into a bank before landing in the culvert.

He said SH94 was a country road, a 100km/h speed zone.

The fire service is still at the scene assisting police.

The serious crash unit is investigating.

Part of SH94 remains closed, and diversions are in place.

Police say motorists should go around the back roads - using Beck Rd, Queen St, and Balfour-Ardlussa.