Three people died after the trailer detached from the Kenworth prime mover on the Wodonga-Yackandandah Road on August 7, 2014.

The court heard the driver had been delivering BP fuel to farms and small service stations in the area when the trailer detached at Staghorn Flat about 8.45am.

It hit two cars.

Heavy Mechanics Pty Ltd serviced the truck and trailer, and prosecutor Andrew Palmer QC said the fundamental question in the case was whether the business "failed to take reasonably practicable steps to ensure that a catastrophic event like that wouldn't happen".

"By failing to take those steps, the company exposed road users, such as the three occupants of those vehicles, to very serious and indeed fatal risks to their health and safety," he said.

"That's fundamentally what this case is about."

Mr Palmer said the company itself, rather than any person or employees, was on trial.

He said evidence would be given about the components.

"Very small movement over time can cause the damage that ultimately can lead to the failing, like the one that occurred on the 7th of August, 2014.

"The question of course is how much wear is too much wear."

IN OTHER NEWS:

Mr Palmer said evidence would be heard about how the truck and trailer were maintained.

"Our case is about the things that we say they should have done but didn't do, which we say would have reduced the risk," he said.

The truck and trailer travelled 354,000 kilometres over a period of three years and two months without its coupling being replaced, the court heard.

Heavy Mechanics took over the servicing of the vehicle and tanker trailer from BMR Truck and Trailer in November 2011.

The court heard after taking over servicing the truck and trailer, to the time of the triple fatal crash at Staghorn Flat in August 2014, the coupling between both components was never disassembled or replaced.

It was last serviced on August 1, 2014, six days before decoupling at Staghorn Flat and hitting two cars.

The prosecution alleges there was progressive wear of the components over time due to looseness.

The court heard there should have been a "snug fit" between a tow eye and block on the trailer.

Andrew Palmer outlined the prosecution case, and said visible wear should have led to the components being disassembled and other steps taken.

"Our case is that if the system we are arguing for ... had been followed, we say that the damage and wear to the coupling would have been detected well before the coupling failed," he said.

"And that's how we say it would have reduced risk."

It's argued the steps taken weren't enough and the servicing inadequate.

"And it's also about the things that the evidence shows they actually did, which we say did not reduce the risk enough," he said.

Evidence will be heard from multiple people, including a mechanical engineer and metallurgist.

Chief mechanic Keith Haire, administrator Michelle Haire and mechanic Richard Trethowan will give evidence.

Mr Haire has entered a not-guilty plea on behalf of the business.

The court heard there would be a strange mix of evidence, given some of it had been given in a previous trial before a different judge.

Recordings, evidence on video link and in-person evidence will be given.

Daniel Gurvich QC will outline the defence case on Thursday.

Judge George Georgiou told the jurors they hadn't heard actual evidence in the matter, which will begin Thursday.

"It's on that evidence you will make your decisions," he said.

Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content: