As the haul truck was leaving a dump and travelling down a ramp, the operator applied the retarder, however, it had no effect.

The operator then tried the service brake, which also had no effect.

The operator noticed there was no air pressure indicated.

The emergency brakes were then applied, which slowed the truck and helped it safely negotiate a bend.

The air pressure returned, and the brakes subsequently worked.

The operator then continued to a digger but noticed the brakes were inoperable again, so proceeded to park up and report the incident.

The NSW Resources Regulator said an investigation found an air conditioner compressor belt drive guard was rubbing on the discharge fitting of the compressor.

"The fitting failed, resulting in loss of system air pressure and loss of the service brakes," it said.

"Under no circumstances should a truck continue to be operated following the failure of a safety-critical system such as braking. Vehicles should be safely parked-up following an incident and not operated until the cause is identified and problem rectified."