He said it was disruptive that health and safety representatives would be able to give notice of suspension of mining operations when there is a risk of serious harm.
The representatives could also call an immediate stop if the risk was immediate.
"It's a pretty powerful part of the act and quite frankly a scary proposition in the hands of an ill-informed or maybe overly-eager representative.''
OceanaGold said in a statement it had been supporting the ministry during the review process and the Reefton mine was still operating.
"The company was made aware that aspects of its operating procedures for tipping waste rock in a localised area of the mining operation were not being consistently practised. We acted immediately to address the issue.
"We encourage worker participation at all of our operations and have health and safety representatives actively involved in the health and safety of all areas of our operations.''
Engineering, Printing and Manufacturing Union director of organising Alan Clarence said the prohibition notice showed OceanaGold's hypocrisy in opposing stronger regulations and greater health and safety powers for workers.
"Mr O'Leary ... told MPs his company was squeaky clean, all the time knowing perfectly well that MBIE had shut down part of his operations due to serious safety breaches.''
He said the latest prohibition notice showed gold mining was a high hazard industry with serious risk of workplace death and injury.
"It took 29 deaths at Pike River for the coal mining industry to stop its lobbying against mine safety improvements. We shouldn't have to wait for another tragedy before the gold mining industry does the same.''
The legislation comes from recommendations by the Royal Commission into the Pike River disaster, which killed 29 workers.
It would establish a standalone Crown agency, WorkSafe New Zealand, governed by a board of up to nine people and reintroduce health and safety representatives in mines.