Heavy Mechanics has pleaded not guilty in a trial at Wodonga Magistrates Court to the charge of failing to ensure people were not exposed to health and safety risks.

The trailer detached from the truck at Staghorn Flat on August 7, 2014, causing the deaths of three other people on the road.

Crown prosecutor Andrew Palmer, QC, gave his closing address on Tuesday, saying the company failed to see the wear on the tow eye coupling during regular services, because it did not detach the truck and trailer for a proper inspection.

The tow eye coupling was in front of him on the bar table as he addressed the jury.

"The accused company failed in what it should have done to make sure the tow eye coupling was safe," he said.

"The failure of the tow eye was the result of a cumulative, progressive wear ... "You can just look to see the rust and degeneration on these parts."

The BP truck and trailer was driven to Melbourne every day to load up on fuel, then returned to the North East where the fuel was distributed to farms and small service stations.

Heavy Mechanics conducted a service every 10,000 kilometres, usually fortnightly, plus an annual service.

Mr Palmer referenced the evidence given by the mechanic, who serviced the trailer six days before the crash, saying the statement that the company was very safety conscious had little weight.

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"It is the evidence of a young mechanic who finished his apprenticeship at the company - you might think he has good feelings towards the company," he said.

"We know that it wasn't 'all good' (as said by the mechanic), it was actually extremely dangerous."

He rejected the idea the decoupling was a sudden event that could not have been foreseen.

The defence will begin its closing argument when the trial resumes on Wednesday.