A truck has caught fire on the M2 motorway at North Rocks, causing lengthy delays for motorists on the first day of the October long weekend.

All eastbound lanes have been closed, with traffic heading into the city backed up 2.3 kilometres at midday on Saturday. Motorists are being diverted onto Windsor Road.

The westbound bus lane was also closed as emergency services attended to the blaze.

The fire broke out in the prime mover of the truck carrying cars about 11am on Saturday. It then spread to four vehicles on board the truck.