Hamiltons Blasting Services made multiple mistakes during the incident at the Burgess Earthmoving quarry in Ettamogah on April 10, 2018.

The explosion flung rocks into vehicles and narrowly missed two groups of people, who had been too close to the blast site.

A man shouted out "incoming" as large rocks were projected towards the 12 people, who took cover behind vehicles.

Rocks hit a grey Volkswagen Amarok, knocking off its bonnet, a Nissan Patrol and a Nissan Navara.

A rock passed close by a man's head before hitting an excavator and cracking a hydraulic pipe.

The District Court recently heard about multiple workplace health and safety failings during the incident.

The business is run by David Steven Hamilton, who founded the company in 2016 and began conducting blasts at the Ettamogah quarry in November of that year.

He failed to notify SafeWork NSW of the incident, which is a legal requirement.

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Mr Hamilton has made multiple safety changes since the accident and the court heard he was "extremely remorseful and contrite about what has occurred".

Mr Hamilton said it was a serious error in judgement to allow the public and quarry workers to watch the blast.

"The incident has had a profound effect on Mr Hamilton and he wants to make amends for what has occurred," the court heard.

Judge David Russell said the company was more culpable over the incident than Burgess Earthmoving, which was fined $235,000 in October.

"It was irresponsible in the extreme for Hamiltons to knowingly permit so many innocent bystanders to be in the danger zone when the shots were fired," Judge Russell said.

He noted Hamiltons had expertise and qualifications in rock blasting which Burgess didn't have, and Mr Hamilton had the ultimate authority over the blast.

"He could and should have declined to do (the blast) while there were any persons in the quarry," Judge Russell said.

Hamiltons must pay $324,000 and Mr Hamilton will pay $22,500, with both to also pay the costs of the prosecution.

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