A general technical fault has been blamed for leaving Swansea bridge stuck open, however further inspections are needed to find out what caused the seven-hour delay.
On Wednesday, the bridge opened for a vessel at about 3pm but the northbound bridge failed to close properly.
Traffic quickly backed up, with northbound drivers unable to continue their journey.
A contraflow was put in place on the southbound bridge, as crews worked to repair the problem and the bridge was reopened about 10pm.
Although the problem was fixed, its specific cause remains unknown.
"Transport NSW engineers are now carrying out further inspections and investigations to find out the cause of the problem," a Transport NSW spokesperson said.
Maintenance work is carried out every six months on Swansea Bridge, with work last carried out earlier this month. The last time the bridge had a fault was more than four years ago, in September 2019.
More than 27,000 road vehicles use Swansea Bridge each day. It opens 2000 times a year, which is around five to six times a day, allowing up to 4500 vessels to pass through annually.
