Cahill Expressway truck rollover causes Harbour Bridge and Tunnel traffic chaos

Motorists have been told to avoid driving towards the city from Sydney's north after a truck rolled over, causing "very lengthy delays".

The truck, which was carrying dirt, overturned on the southbound on-ramp from the Cahill Expressway to the Domain Tunnel just before 1.45pm.

The truck, which was carrying dirt, overturned on the Cahill Expressway. Photo: Seven News

Its rear trailer tipped sideways, tipping dirt and debris onto several lanes.

As a result, the Cahill Expressway and the Sydney Harbour Tunnel have been closed southbound, while northbound traffic on the Eastern Distributor is congested.

The Transport Management Centre (TMC) said traffic is "extremely heavy" after the crash across parts of the city, including the Sydney Harbour Bridge.

Motorists are queued for six kilometres from the Harbour Bridge to Crows Nest.

"As a result, motorists should avoid driving towards the city from the northern beaches, lower north shore and The Hills areas," a TMC spokeswoman said.

"The Sydney Harbour Bridge is open, but traffic is extremely heavy back to Crows Nest.

"Motorists already using the Harbour Bridge are being diverted onto York Street, and should allow plenty of extra travel time and expect very lengthy delays."

Some buses heading to the city aren't travelling over the Harbour Bridge and are ending their journeys at North Sydney station.

Commuters were advised to catch a train.

Just before 2.30pm, the truck's trailer was put back onto its wheels by a heavy tow truck and emergency services.

However, the delays are expected to continue for some time as the dirt on the road is cleared up.