The accident happened on the Castlereagh Highway on Tuesday morning.

Lightning Ridge Police senior constable Brett Henderson-Smith said the men were about 20km from Lightning Ridge travelling north when a sheep came onto the highway, causing the driver to swerve to avoid hitting it.

As the car had swerved it left the road surface on the left-hand side, and after hit something puncturing one of the tyres.

As a result the car swerved back onto the road and across to the other side before ending up on its roof.

Snr Const Henderson-Smith said more damage would have been done to the vehicle, and the driver and passenger wouldn't have been able to leave the scene unscathed, if they had been travelling any faster.

"They (victims) were only doing about 100kms, but if they were travelling at the speed limit of 110km/h or more, the car would have certainly rolled a couple of times," he said.

State Emergency Services crews and the fire brigade attended the accident and assisted moving the car from the scene.