Lisa Turner and her four-year-old son Jack Wallace died when their car was hit by a decoupled petrol tanker trailer on the Wodonga-Yackandandah Road on August 7, 2014.

Peta Cox also died when her vehicle was struck.

Jurors in the trial of Heavy Mechanics Pty Ltd, which serviced the truck and trailer 62 times before the crash, on Monday returned a verdict of guilty.

The Wodonga company last serviced the trailer six days before the fatal incident, with the court told the servicing was inadequate and failed to detect wear on the failed components.

Family members, who had sat through two trials and a string of other court matters, reacted with relief when the verdict was handed down nearly seven years after the tragedy.

Jurors were not told the identities of those killed, nor the fact family members were present in the court during the trial.

Damien Wallace, who lost his wife Lisa and son Jack in the incident, said it was a relief but nothing would bring the pair back.

"This is never going to be justice for what happened," he said.

"The verdict of guilty gives us at least something."

Heavy Mechanics owner Keith Haire breathed heavily when the verdict was handed down.

Irma Turner lost her daughter and grandson in the crash.

She wiped away tears when the verdict was handed down and when speaking of the pair.

Her daughter was a nurse based in Wodonga and her grandson was set to start school the year after the crash.

Ms Turner said lives had been destroyed by the crash, but believes the verdict will allow the family to move on after a "horrendous" legal process.

"It's been a long road," she said.

"It's the worst thing for a mother to lose a child and a grandchild.

"So I'm so happy they've at least found what's caused it, it wasn't an accident.

"At least another family won't have the same tragic thing happen to them."

She wants to see the type of towing coupling involved, which relies on a nut to hold everything in place, banned from use.

"The verdict is going to allow us to move forward and remember all the good times without reliving the accident all the time," Ms Turner said.

"We haven't had closure legally, we'll never have closure over their deaths, but at least we'll be able to move forward and remember the good times."

The matter will return for sentence at a later date.

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