A failed nut found at the scene of a triple fatality is at the centre of the case, a court has been told.
Three people in two vehicles died at Staghorn Flat on August 7, 2014, after a truck and BP petrol tanker trailer decoupled and hit the cars on the Wodonga-Yackandandah Road.
The Wodonga County Court was told on Tuesday Heavy Mechanics Pty Ltd serviced the trailer six days before the crash.
Director Keith Hare has entered a not guilty plea to one charge on behalf of the company.
It's alleged the company failed to ensure people were not exposed to health and safety risks.
Prosecutor Andrew Palmer showed jurors images of failed components on the truck and trailer, including a nut found on the side of the road near the crash scene.
"That nut, small object as it is, is absolutely crucial to this case," he said.
"The fact this nut is on the road is a big problem."
If not properly tightened, Mr Palmer said there could be "catastrophic consequences".
The court heard manufacturer JOST recommended the nut be tightened to 500Nm of torque.
"Everything depends on that nut," he said.
The court heard a tow eye coupling was replaced more than three years before the crash, and the trailer was serviced at the Kane Road business on August 1, 2014, six days before the crash.
Detective Senior Constable Philip Frith said the trailer travelled 356,000 kilometres from June 17, 2011, when the tow eye was replaced, to the date of the crash.
The truck and trailer were serviced 68 times in that period, he said.
A company that previously worked on the trailer replaced previous tow eyes after an estimated 100,000 kilometres and 200,000 kilometres of travel.
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Mr Palmer said experts would tell the court the tow eye had moved around and worn over time.
"The real issue in this case is about whether the service and maintenance ... was adequate," he told jurors.
Mr Palmer asked if the company had done enough to discharge its obligation to make sure the coupling was safe.
He said there was "significant wear to the thread of the shaft".
The case was not about the deaths of the three people, Mr Palmer said, but the responsibility of the business to service and maintain the trailer.
He argued the business failed to take reasonably practicable steps to ensure such a crash would not happen and expose road users to health and safety risks.
Jurors viewed images and a diagram of the crash scene.
They also read through job sheets sent to Heavy Mechanics by a BP subsidiary.
Damage would have been visible to anyone inspecting the tow eye, the prosecution argued.
Defence barrister Daniel Gurvich, QC, said the case would centre on whether the company had done what was "reasonably practicable".
"Did they do what any other decent qualified mechanic would have done" in their place, he asked.
Mr Gurvich told jurors they should keep an open mind.
"The accused is presumed innocent," he said.
"Every accused is, man, woman or company."
The prosecution alleges there was a risk if the tow eye coupling wasn't properly serviced and maintained, it could fail and the trailer decouple from the truck and expose road users to a risk of serious injury or death.
The prosecution argues Heavy Mechanics should have taken steps including performing a visual inspection to check the block and tow eye coupling were in a serviceable condition, and check the tow eye coupling for movement.
It's alleged in the event any signs of movement, damage or wear were detected, the tow eye coupling should have been disassembled in full or in part, further inspected, and depending on the condition of the coupling, replaced or reassembled in accordance with the manufacturer's specifications.
Detective Senior Constable Frith said he had checked records from Heavy Mechanics and BP, nearly dating back to the start of the life of the truck and trailer.
He said he had looked through a "significant number of documents" during the investigation.
The court heard the Kane Road business took over the servicing from November 15, 2011, from BMR Truck and Trailer, which had operated from the same site before changing hands.
The court heard someone from BMR Truck and Trailer would give evidence, along with staff from Heavy Mechanics, two experts and a WorkSafe inspector.
The driver of the truck told police at the scene he didn't know what had happened.
Leading Senior Constable Guy Tinsley said he spoke to the driver of the BP petrol tanker, Patrick Daley.
He tested the man for drugs and alcohol, which gave negative results, and also looked at his log book.
"That was all correct," the highway patrol member of about 30 years said.
The driver said he "saw it come off" and travel down the road towards the cars.
Leading Senior Constable Tinsley said he had arrived at the Wodonga-Yackandandah Road at 9.09am on August 7, 2014.
He spoke to multiple people at the scene.
He asked the driver for his manifest - which details where his load was headed - which he provided.
The trial before Judge Christopher Ryan continues on Wednesday.