A driver has died after suffering a suspected medical episode and crashing on the Sydney Harbour Bridge.

Emergency services were called to the bridge on the Bradfield Highway about 12.40pm on Saturday following reports of a two-vehicle crash.

NSW Ambulance said paramedics treated the driver, a man aged in his 70s, at the scene before transporting him to Royal North Shore Hospital in a critical condition.

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He later died in hospital and is yet to be formally identified.

The four occupants of a second vehicle were also assessed, but none required treatment or transport to hospital.

Multiple emergency vehicles were at the scene. Credit: 7NEWS

Paramedics were seen treating a patient. Credit: 7NEWS

Images from the scene show multiple emergency vehicles surrounding the crashed cars on the bridge.

Police have commenced an investigation into the circumstances surrounding the crash, and a report will be prepared for the coroner.

The crash has affected northbound traffic, with long queues forming on the bridge.

Southbound lanes remain largely unaffected.

Long queues formed as paramedics tended to those caught up in the crash. Credit: 7NEWS

Northbound lanes were heavily congested following the crash. Credit: 7NEWS