A spokesperson for fire crews at the scene said the crane came off the semi after its driver was forced to brake suddenly, after being cut off by the car travelling in front of it.

After the driver hit the brakes, the crane slid sideways.

Emergency services and Bathurst Regional Council staff rushed to the scene; the truck blocking one westbound lane of traffic causing traffic chaos at the eastern entrance to the city.

Emergency crews arrived to find the crane had fallen off the passenger's side of the low-loader trailer and was balancing next to the guard rail running along the bridge.

Emergency crews were faced with a big challenge in planning to remove both the truck and its load, it was expected both westbound lanes would be closed and traffic reduced to one lane each way along the eastbound lanes of the bridge while the salvage operation was underway.

However, late Tuesday one westbound lane remained open, as did both eastbound lanes.

Authorities only had to close both westbound lanes for a short period while the truck was removed from the site.

Fire fighter Doug Fisk, from Fire and Rescue NSW said fire crews remained on site during the salvage operation due to contamination risks to the nearby Macquarie River.

He said the crane contained around 100 litres of diesel and 100 litres of hydraulic fuel which were leaking slowly from the overturned unit.

"As it's right above the Macquarie River where here to make sure the leak is contained," he said.

Fire Fighter Fisk said the salvage operation had been a complex one.

"Initially the truck carrying the crane had to be removed from underneath the load. Another crane was then called to slide the truck out and they were able to shimmy it out from underneath."

A larger crane was en-route to the accident site late Tuesday afternoon to assist in the operation.