A car on fire near Birkenhead off-ramp has caused traffic chaos on the Harbour Bridge.

Northern Fire Service spokeswoman Megan Ruru said two crews attended the incident about 12.20pm.

The car has now been extinguished and firefighters have left the scene.

The incident occurred just before the Stafford Rd off-ramp on State Highway 1.

Northern Police spokesman Chris Tate said the cause of the blaze was not yet known.

Police had received plenty of calls about the blaze, he said.

The New Zealand Transport Association tweeted motorists should expect delays just north of the Harbour Bridge at 12.40pm.

Lanes were blocked due to an incident, the association said.

Twitter user Rick Shera replied: "You're not joking. Both north bound clip ons closed. Curran St on ramp crossing to middle. #slooooooooooooow".