The highway was partially closed south of Breadalbane, between Gunning and Goulburn, about 3am on Tuesday morning after the B-double truck caught fire.

All southbound vehicles were being detoured towards Canberra however both lanes had reopened by 9.30am.

The road was closed intermittently while the vehicle was removed.

Motorists were advised to exercise caution.

The truck fire also sparked a grass fire on the side of the road, which was quickly extinguished by firefighters.