Moranbah North mine operator Anglo Coal is fighting a charge of failing to discharge health and safety obligations causing death over the February 2019 incident.
It was a hot day at the site inland from Mackay, and Mr Hardwick had stopped the vehicle on an underground roadway, known as a drift, to access water to cool the engine, prosecutor Joshua Trevino KC said on Monday.
Shortly after, the grader descended backwards out of control over about 250 metres before it hit another vehicle used for transporting mine workers, known as a drift runner.
"It's not known exactly where Bradley Hardwick was when the grader started its backward descent," Mr Trevino told Brisbane Magistrates Court.
"Maybe he attempted to jump off the machine as it moved down the drift … it was a long and frightening way down to the end of the tunnel."
Both vehicles slid a short distance before coming to rest on the side of the drift, the court was told.
After the collision, workers in the drift runner walked up the roadway and found Mr Hardwick lying on the ground with his feet toward the surface of the mine.
He had suffered fatal injuries.
"There is no direct evidence to explain how Bradley came to move from manhole 20 alive … to near manhole 30 and dead," Mr Trevino said.
The circumstantial inference that can be drawn is it was the result of some interaction with the uncontrolled grader, he told the court.
At the centre of the case against Anglo is the grader's park brake, and the apparent inability to test it independently of its service brakes.
Mr Trevino argued that Mr Hardwick couldn't check if the park brake was functioning properly, and it appeared he had carried out a pre-start inspection on the grader.
"The operation of the service brakes may mask the inoperability of the park brake, it certainly appears to be the case for Bradley," he said.
"The box for the park brake is ticked, indicated that he had carried out the relevant test and the grader had passed that test, but we know with the benefit of hindsight that ... at that time the park brake was defective.
"It was defective because its friction plates were worn down."
The service brakes use air circuits and operate like conventional brakes on a car, the court was told.
A loss of air pressure can cause the service brakes to release, which can be caused by a number of factors.
Anglo Coal's lawyer Saul Holt KC said much of the case material was agreed, including that the park brake plates were worn.
But Anglo operated consistently with the instructions for testing the grader given by the original equipment manufacturer (OEM), he said.
"It is no part of Anglo's case that the fact the OEM has obligations under the act relieves Anglo of its obligations," Mr Holt said.
That does not mean the conduct of the OEM, the information provided to Anglo and the design of the "somewhat unique breaking system" is not relevant, he told the court.
Evidence will show that "for all money" the OEM presented as competent to Anglo and the mining regulator, but had behaved "with what can only generously be described as gross negligence," he said.
The defence case is also expected to highlight doubt on whether the grader caused Mr Hardwick's death, or if there was another source of sufficient force.
The case continues.
