An excavator operator accused of killing a man after a piece of machinery fell and hit him in the head at a worksite, "panicked" and deliberately tampered with the scene to cover up the truth, a court has heard.

Key points: Prosecution tells the court Neil Norris had not attached a safety pin to an excavator that led to 1.2 tonne shears detaching and striking Peter Tullett in the head

Prosecution tells the court Neil Norris had not attached a safety pin to an excavator that led to 1.2 tonne shears detaching and striking Peter Tullett in the head Mr Norris quickly fixed the machinery before anyone could notice, the court hears

Mr Norris quickly fixed the machinery before anyone could notice, the court hears The prosecution says Mr Norris has been licensed in the equipment for six years prior to the incident, so was well aware of relevant safety checks

Neil Edward Norris pleaded not guilty to manslaughter at the beginning of his Supreme Court trial in Brisbane on Monday.

Peter Tullett, who was a mechanical fitter, suffered fatal head injuries at a Springfield Lakes construction site in April 2017 and later died in hospital.

The prosecution claimed Mr Norris had not attached a safety pin to the excavator that resulted in 1.2 tonne shears detaching and striking Mr Tullett in the head.

In his opening address, prosecutor Philip McCarthy said Mr Norris quickly fixed the machinery before anyone could notice.

"Neil Norris determined to deliberately lie about how the incident happened from the outset," Mr McCarthy said.

"He told lies to the workmates who had attended to help Peter … he told lies to the first police officers … he told lies to the inspector from Workplace Health and Safety [WHS].

"Not only did he lie, but in the short time before workmates arrived to assist Peter, he and his boss … ensured the mechanical shears were reunited with the quick hitch of the dipper arm.

"He and his boss ensured that the manual safety pin was inserted … before workmates arrived."

Mechanical fitter Peter Tullett suffered fatal head injuries and later died in hospital. ( Facebook )

The court heard Mr Norris originally told investigators at the scene that Mr Tullett was on top of a nearby chipper and the pair was trying to remove a heavy guard cover that suddenly released and hit him.

Jurors were told Mr Norris confided in a colleague a few weeks after the incident and told her the shears had come off and the safety pin was put back in afterwards.

The prosecution said when speaking with police months later, Mr Norris told them that the shears fell off while the excavator was trying to pull the cover off the chipper.

"Mr Norris told the officers that he should have made sure the safety pin was in," Mr McCarthy said.

"[He] informed the police that he called [his boss] down and in his words, in just a 'mix of adrenaline rush and panic and not knowing what the f*** to do, we just put the pin back in'."

The prosecution said Mr Norris was "no novice" and had been licensed in the equipment for six years prior, so was well aware of the relevant safety checks.

The trial continues.