Contract worker Mark Brilley had a heart attack at the Cobar mine, in central western NSW, in April 2017.

The heat, inadequate ventilation and recirculation of toxic diesel gases subjected his body to severe physiological stresses that caused his acute myocardial infarction and sudden death, according to a cardiologist

Peak Gold Mines Pty Ltd pleaded guilty in the NSW District Court to failing to comply with its health and safety duty, thereby exposing Mr Brilley to a risk of death or serious injury.

Mr Brilley, 36, was an employee of Jemrok Pty Ltd, a mining services contractor engaged by the operator to undertake underground development works.

Judge Andrew Scotting on Thursday fined the company $480,000 and ordered it to pay the legal costs of the prosecutor, SafeWork NSW.

"The offence is one of considerable objective gravity," the judge said.

"The risk was obvious and well known to the offender."

The operator knew the impeller replacement works were likely to adversely affect the mine's ventilation and could result in dangers to the health and safety of the workers engaged underground while those works took place.

"The likelihood of the risk occurring was high," he said.

The reduction in the extraction of air from the mine was almost certain to increase the concentrations of contaminants in the air, posing a risk to anyone working underground.

"The steps that could have been taken to avoid or minimise the risk were simple steps that were known to the offender and involved little expense or inconvenience," the judge said.

'"The risk assessment conducted by the offender was wholly inadequate and was based on engineering judgment that had the potential to be fallible and proved to be so."

The company could have made adjustments to the ventilation system and/or the arrangements to withdraw workers while the impellent replacement works were undertaken.

"The steps taken by the offender fell well short of the standard of care required, to ensure the health and safety of the workers."

A victim impact statement from Mr Brilley's mother revealed she was at the mine on the day and as a result suffered "the trauma of the death of her son in close quarters".

Her statement and others were "a moving tribute to a much loved son, partner, father, brother and friend".

The judge said the penalty must provide for general deterrence, noting the community is entitled to expect that employers will comply with safety requirements.

It also must provide for specific deterrence for Peak Mines, which continues to operate a business that poses a significant risk to the health and safety of its workers.

"I note that the offender has taken extensive steps to improve its safety systems since the incident," he said.

The company had no previous convictions, had good prospects of rehabilitation, had demonstrated remorse and paid some money to Mr Brilley's family in reparation.

Australian Associated Press