Police said a potential fireball was narrowly averted when a broken down fuel tanker in the west-bound emergency lane of the Western Fwy, near the Pentland Hills Rd, was hit by another truck, causing fuel to spill near the road.

The BP-branded tanker was carrying an estimated 27,000 litres of diesel and unleaded fuel.

Another truck hauling heavy machinery ploughed into the side of the tanker, causing several hundred litres of fuel to spill.

Country Fire Authority region 14 operations officer John Anderson said the tanker was separated into five compartments.

The tear occurred along the fifth compartment, which had most of its load emptied at Bacchus Marsh earlier in the morning.

"We are probably lucky it (an explosion) was averted," he said.

"We are lucky the compartment that was split emptied in Bacchus Marsh. We have managed to ascertain that not much diesel has spilt, that it's mainly hydraulic fuel that has come out."

Heavy haulage equipment was used to clear the scene.

Traffic in both directions was was diverted through Myrniong as the trucks were separated.

Bacchus Marsh Senior Sergeant Damien Jones said it was likely at least one west-bound lane of the freeway would remain closed well into last night.

`It's not worth the risk," he said.

"If the truck that collided into the side of the tanker hnd hit it straight on or to the back of it, it could have been a very bad result for everybody."

A 37-year-old Edithvale man is facing careless driving charges from the incident.

The truck was owned by Reliance Petroleum, which carts fuel for BP.

Reliance Petroleum did not return calls yesterday.