Major traffic jams stretching back 10 kilometres were experienced during Sydney's peak hour on Thursday when a car overturned in the Sydney Harbour Tunnel, forcing the closure of the tunnel's south-bound lanes.
The crash occurred about 7.30am and resulted in traffic queues snaking back to the Lane Cove Tunnel and to Seaforth, and delays of up to an hour on Sydney's buses on the lower north shore.
Police from the Harbourside local area command said the crash occurred when a driver who was not feeling well attempted to merge into the tunnel's left break-down lane.
"Unfortunately it would appear he misjudged and entered towards the end of the break-down lane and drove up a safety wall which flipped the car onto its roof," police said.
A doctor who was in a car behind stopped and treated the man, aged in his 50s, until paramedics arrived. The man was taken to hospital with a minor neck injury.
Police said there was a large amount of oil on the road, and some minor damage to the tunnel's wall needed to be repaired.
Cars that were caught in the tunnel immediately behind the crash were allowed to drive through, but the tunnel was then closed to other traffic until just after 9am.
The closure forced all south-bound traffic onto the Sydney Harbour Bridge, which struggled to cope with the influx of extra vehicles.
The Transport Management Centre (TMC) warned motorists to expect significant delays.
By 10am, traffic was still queued back five kilometres to Willoughby Road, a TMC spokeswoman said.
Buses travelling towards the Harbour Bridge were also delayed significantly. Some were taking the Pacific Highway instead of the Gore Hill freeway.
Commuters in affected areas were told they could use their bus tickets on trains and ferries instead.
Police are continuing to investigate the cause of the crash, and have urged any witnesses to contact Mosman Police Station or Crime Stoppers on 1800 333 000.