A head-on collision on the Sydney Harbour Bridge closed multiple lanes of traffic heading into the city during peak hour, with commuters feeling the effect up to 20km away.

The crash occurred about 7.45am, causing traffic to pile up as emergency services respond to the incident.

“The Harbour Bridge is closed citybound due to a multi-vehicle crash on the deck of the bridge,” Live Traffic said in a tweet just before 8am.

Authorities worked quickly to clear the site, with all lanes reopened within 30 minutes and traffic beginning to flow again by 8.15am.

“While all lanes are now open, traffic remains very heavy approaching the Sydney Harbour Bridge,” a Transport Management Centre spokesperson said.

“Citybound traffic is heavy back to the Lane Cove Tunnel while northbound traffic is queued across the Anzac Bridge back to Victoria Road.”

While the crash was cleared within just 30 minutes, flow-on effects are affecting morning commuters travelling into the city from all directions.

Motorists are still advised to allow plenty of additional travel time, and citybound buses on the Sydney Harbour Bridge are delayed.

Drivers are being urged to use the the Harbour Tunnel to avoid significant delays as traffic banks up in both directions on the bridge.

“Traffic is very heavy both ways, expect delays and take care with emergency services and tow trucks on site,” Live Traffic said in a second tweet.

Aerial shots of the Harbour Bridge shows gridlocked traffic stalled in at least three lanes heading towards the city.

One northbound lane is closed on the Harbour Bridge as well as two citybound lanes.

The Cahill Expressway and the bus lane also remain open.

NSW Ambulance paramedics, firefighters and transport authorities are on the scene.

There have been no injuries reported.