Traffic piled up this morning after an oversized truck blocked the Sydney Harbour tunnel in peak hour.

The truck, which was too high for the tunnel, was headed southbound when it found itself stuck.

Live Traffic first reported the incident at about 7.45am, saying the southbound lanes were closed and urging people to use the Harbour Bridge instead.

The tunnel has since been reopened, but Live Traffic warned it would take some time before traffic returned to normal.

At one stage it had backed all the way up to the Lane Cove tunnel.

Meanwhile, in the city's west, two of the four westbound lanes of the M4 are closed before James Ruse Drive at Auburn after a crash, with drivers in the area urged to take care.

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