Jurors were close to considering their verdict in the Heavy Mechanics trial after about two weeks of evidence.

The company serviced a BP truck and tanker trailer before it decoupled on the Wodonga-Yackandandah Road in August 2014.

It hit two vehicles, killing four-year-old Jack Wallace and his mother Lisa Turner, and Peta Cox.

Jack's grandmother, Irma Turner, wiped away tears after learning the jury would be discharged without a verdict.

Judge Christopher Ryan on Friday told the 12 jurors "at the 11th hour, an issue of legal significance in relation to the fairness of this trial was raised".

He said it was a complex issue, which limited the argument the defence could put forward to the jury.

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"An incurable irregularity has occurred and accordingly this trial must come to an end without you returning a verdict," he said.

"Accordingly, I am required to discharge you today in accordance with established legal principles."

Damien Wallace, who lost his wife and son in the crash, held his face in his hands and other family members could be heard saying they just wanted the matter finalised.

The judge said it was important the defence receive a fair trial.

Lisa Turner's father and Jack's grandfather, Gerry Turner, was at times distressed during the trial.

"We're not too good, horrible in a way, because we've still got no closure," he said.

The case will return to court on November 28 and another trial may be held next year.

"It is plain to me that you have conducted yourself as conscientious jurors," Judge Ryan told the jury of eight men and four women.

"I appreciate that you must be experiencing a great deal of frustration and that you may form the view you have wasted many days of your life.

"Let me assure you that is not the case because all the evidence has been recorded, and a new trial of this matter is inevitable.

"It will be run more efficiently than this trial was."

He held up his hand and gestured, telling the jurors they were "this close" to going into the jury room to consider their verdict.

"On behalf of the community, I want to thank you for the effort you have made ... despite the fact you are discharged without verdict," he said.

The business faced a charge of failing to ensure people weren't exposed to health and safety risks.

The prosecution alleged the Kane Road business hadn't adequately serviced the truck and trailer, with expert evidence about wear on the failed components.

Defence barrister Daniel Gurvich, QC, argued the servicing had been appropriate and in line with industry practice.