A truck driver who crashed on the Cahill Expressway, causing traffic gridlock across Sydney's CBD in scorching conditions on Wednesday afternoon, had been disqualified from driving until 2019 and was using a fake driver's licence at the time, police say.

The 27-year-old man was charged with multiple driving offences following the crash near the expressway's exit ramp to the Domain Tunnel.

The crash forced the closure of south-bound lanes of the expressway and Sydney Harbour Tunnel for hours.

Police said the man was driving a Volvo prime mover south on the expressway about 1.20pm when, as he attempted to change lanes, his rear trailer mounted the centre concrete divide and struck the traffic gantry over the exit ramp.