An open window has saved a man who was inside a car that plunged into Mount Gambier's volcanic Blue Lake late last night, police say.

A Mitsubishi ute smashed through a guard rail and fence near the Blue Lake lookout about 11:30pm, before hurtling down a cliff towards the water.

Police said it took a rescue boat several hours to get the uninjured 22-year-old man down from the steep slope and into the boat.

Senior Constable Mick Abbott said it was a miracle the man got out of the car before it hit the water, even if it was by accident.

"From what I understand, speaking to the investigators at the scene, the driver's side window was actually down so he was actually thrown out of the car as he was going down this steep embankment — so he's actually quite lucky that happened," Senior Constable Abbott said.

Blue Lake sits in a crater created by a volcanic eruption about 5,000 years ago.

Its water shines blue in spring and summer because it is naturally cleaned as it moves through the limestone aquifer beneath it.

Water supply switched from lake to bores

SA Water has switched Mount Gambier's water supply from the lake to the local borefield until the ute is removed.

The barriers the ute crashed into before plunging into Blue Lake. ( ABC News: Lucy Robinson )

In a statement, the company said the water "remains safe to drink" but customers may notice it tastes or looks slightly different.

General manager Mark Gobbie said it could be some time before locals were back on lake water.

"We're looking at whether we can actually float the vehicle, [but] unfortunately lifting with a crane is quite complex," Mr Gobbie said.

"Where the vehicle is at the moment it's actually trapped under some rocks as it's come down the embankment."

The man has gone to hospital for a check-up as a precaution and for police to check for any alcohol in his blood.

Emergency services are still working out how to extract the now fully-submerged ute.

"The car is actually pretty much completely submerged now, right at the foot of the cliff or the steep embankment, so it's going to take some time before the car's actually out of the water," Senior Constable Abbott said.

"I'm sure how they're going to do that but it'll take some time."