Stop-slow alternating traffic conditions are in operation near the site of the crash. Local council workers are at the scene, two kilometres north of Giro Rd.

Two trucks travelling east on the Thunderbolts Way collided about 4pm on Wednesday.

A spokesman for the Hunter’s Westpac Rescue Helicopter Service told Fairfax News the two trucks were travelling in the same direction along Thunderbolts Way when the truck behind, which was pulling a load of scrap metal, notified the other truck driver after his brakes began to malfunction. 

"The truck driver behind radioed the driver in front to say I’ve lost my brakes and the truck in front tried to block him to slow him down," the spokesman said.

He said a trail of debris more than 100m long had been left along the road and it was clear the trucks had attempted to reduce their speed by going up the high side of the embankment several times. 

"Unfortunately, at some point they’ve both gone off the low side and over a pretty steep 45 degree incline," he said.

The driver and sole occupant of one of the trucks, a 32-year-old Karuah man, died at the scene.

A 47-year-old man and 51-year-old woman travelling in the second truck were airlifted to John Hunter Hospital for treatment of non-life threatening injuries.

The 47-year-old truck driver will also undergo mandatory blood and urine tests.

Police from Manning-Great Lakes Local Area Command and the Port Macquarie Crash Investigation Unit are investigating the collision and will prepare a report for the coroner.

The stricken trucks are expected to be retrieved in the coming days while Gloucester Shire Council will undergo a major clean-up operation of the road.

WEDNESDAY: Thunderbolts Way is closed to all traffic in both directions after a serious two-truck crash on Wednesday afternoon.

The crash happened about 4.30pm when two trucks went off a cliff while travelling on a steep, straight section of the road about 2km north of the Giro Rd turn-off.

Emergency service crews were struggling to reach both trucks and their drivers who were trapped more than 50m down a steep cliff.

Specialist climbing gear had to be brought in by the SES to reach the trucks.

The Westpac Rescue Helicopter flew to the Giro Rd turnoff shortly after the accident and remains on standby.

Dozens of emergency service personnel remain at the scene.

It is not known at this stage how many people were involved in the crash or how bad their injuries may be.

Police have advised that the road may not reopen on Wednesday night.

All motorists are advised to seek alternate routes.

Motorists are advised to allow additional travel time and exercise caution passing the incident.