Police say oil may have been deliberately put on a major thoroughfare in the Royal National Park south of Sydney, causing vehicles to run off the road.

Three cars ran veered off Sir Bertram Stevens Drive and into trees because parts of the road had been covered with oil.

Three men were taken to hospital with minor injuries.

Inspector Matthew Francis says police are investigating how the oil came to be on the road.

"It may appear that the oil has been intentionally placed on the roadway," he said.

"Just by the pour patterns of the oil and the area which it covered on the roadway wasn't consistent with it having been leaked from a vehicle."

The road has been cleaned but motorists are being warned to be careful in the area.

"The national park particularly on a Sunday, we always expect high volumes of traffic travelling down towards the beaches but it's a common route for motorcycle and car enthusiasts that like to use their vehicles, to use them," Inspector Francis said.

"Particularly given that there's been a quantity of oil on the road, we just ask that they take care so that we don't have any further incidents."