A major search operation is under way on a highway north of Sydney after witnesses reported seeing a vehicle crash off a bridge before being submerged in water on Monday night.

Traffic was disrupted in both directions at the bridge, which spans the Hawkesbury River, after the crash occurred at Brooklyn about 9.15pm on Monday.

Emergency services arrived at the scene, about 50 kilometres north of Sydney, to find the bridge's guard rails significantly damaged.

NSW Police officers, NSW Ambulance paramedics and firefighters remained at the scene well into the night, as a police boat circled beneath the bridge looking for the vehicle and any potential survivors.