The St Clair man was heading west about 8.45am when the trailer skidded off the edge of the road about 1km up the pass.

The trailer, while still attached to the truck, flipped on its side, dumping its load of 20 tonnes of cattle feed on to the side of the mountain.

The truck remained upright.

The pass was blocked to west-bound traffic for two hours while police awaited the arrival of heavy vehicle-lifting equipment to right the trailer.

The National Parks and Wildlife Service organised for a vacuum truck to visit the site due to environmental concerns about the cattle feed seeping into the forest soil, and the clean-up delayed traffic until 3.30pm.

Macquarie Pass is renowned for accidents, but usually it is motorcyclists who come unstuck. Authorities last year branded the road a motorcycle black spot after 144 accidents and six deaths in five years.

A spokesperson for the Roads and Traffic Authority said road safety and traffic management on the pass was monitored regularly.

"Narrow sections of the road are signposted and double-line markings are in place to improve safety and reduce the speed of vehicles on the pass," the spokesperson said.

"Vehicles up to 19m long, including B-doubles, are legally able to use Macquarie Pass."