A semitrailer overturned on the M1 Motorway (Mount Ousley Road) at Cataract about 8.45am, its trailer and laden shipping container blocking both southbound lanes. 

The truck cabin remained attached, arcing up towards the roadside tree canopy and pushing the driver four metres into the air, his seat tipped 90 degrees backwards. 

The crash came on the final day of the school holidays and four days after the closure of Bulli Pass, which is undergoing two months worth of rockfall stabilisation works. 

It would be four hours before the motorway would re-open. Meantime, gridlock backed up for eight kilometres.   

The southbound lane of the Pass was temporarily re-opened to allow some motorists caught in the debacle a way out.

Parliamentary secretary for the Illawarra Gareth Ward said the move was possible because most of the "heavy work" on the Pass was yet to begin. "We always allow for emergency contingencies," he said.

The MP stressed the capacity for reopening the road at short notice was "not something that can be permanent".

The crash occurred about three kilometres north of Picton Road turnoff. 

Police say the truck driver was negotiating a slight bend in the motorway when he lost control. 

The 55-year-old male driver was uninjured. He freed himself with the help of other motorists and did not require the assistance of NSW Ambulance paramedics. 

One of two northbound lanes was closed to aid the emergency services response, causing delays in that direction. 

Lawrence Hargrave Drive was also congested between Clifton and Thirroul as motorists sought an alternative route. This gridlock continued into the evening after a car fire sparked a bushfire at Waterfall, prompting the closure of the M1 there.