A fuel spill in the Sydney Harbour Tunnel is causing chaos for peak hour traffic heading into the city this morning.

The Transport Management Centre says a truck travelling in the south dropped its fuel tank shortly after it entered the tunnel around 6:00am (AEDT).

The tank ruptured spilling diesel fuel for over 200 metres just past the tunnel's southbound entry point.

David Wright from the Transport Management Centre says both southbound lanes are closed.

"There's been a pretty excessive diesel spill in the tunnel," he said.

"Now I'm just watching it on camera at the moment and the fire brigade and harbour tunnel crews are on site.

"They're using what they call dry saw which is like an industrial strength kitty litter to try and absorb as much diesel as possible.

"Diesel is exceptionally slippery and being a fuel it's also very very dangerous."

Southbound motorists are being diverted onto the Sydney Harbour Bridge but lengthy delays are being experienced.

Drivers are being advised to delay their trip or use Victoria Road as an alternative.

There are also delays for drivers heading to the city via the F3 after a car crashed just before the Berowra interchange.

One person was taken to hospital with minor injuries.

Rail Chaos

Trains have been suspended on the south line between Campbelltown and Ingleburn as crews repair overhead wires at Campbelltown.

Buses are expected to replace trains later this morning.

There are also flow-on delays on the Airport and East Hills line and the South Line.

Passengers are being warned to allow plenty of additional travel time.