Charges against PML had previously been dismissed.
In sentencing, Judge Geoff Rea said the company had safety policies, but "standards were allowed to slip, leading to the injuries of these two people".
"In the end, I have determined both charges have been proved on the basis of a failure to provide adequate supervision," the judge said.
"Unfortunately, where the rubber met the road, if that's the right way of putting it, the supervision fell down.
"There was an extremely thorough theoretical model for safety of employees. Unfortunately the theory, at least over the period of time covered by these two charges, did not translate to actual supervision on the job."
However, he did not feel the company had a "defiant" approach to safety standards and it had a clean record.
"They have employees numbering in the thousands ... this is the first time that they have been convicted of an offence or offences such as these."
A statement from OCS Limited's managing director James Fletcher said the company was pleased its record had been recognised and safety remained its number one priority.
"We were sorry that members of our staff had been hurt in these accidents, as safety is a key priority for us and we treat safety concerns very seriously," he said.
"Following these accidents we increased the level of formal supervision on our sites.
"We also discontinued labour-only contracts where we have no control over the provision of chemicals and equipment."
The Department of Labour, which brought the two charges of failing to stop exposure to hazards, and the defence also agreed on reparation of $5000 to be paid to each victim.