A crane has crashed in a major Sydney tunnel, bringing traffic to a standstill.

The crane crashed about 11.30am on Tuesday in the M5 Mwy at Kingsgrove.

It is currently laying on its side.

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The entire motorway was closed for some time, following the initial closure of just the eastbound lanes.

Westbound lanes have now reopened but all citybound lanes of the M5 East remain closed.

Motorists are advised to avoid the area and can consider using the D5 detour instead in both directions.

Paramedics have treated a man in his 30s and taken him in a stable condition to St George Hospital.