Some workers for the New South Wales prison system have been awarded $650,000 in back pay.

The Health Services Union (HSU) says it will effect about 250 workers such as counsellors, social workers and caterers, who were not paid a environmental allowance for working in a prison.

The union's Gerard Hayes says the Industrial Relations Commission (IRC) found that for more than a decade, employees were entitled to be paid extra.

"I think it's fair to say they're difficult conditions," he said.

"Certainly different to what most of our members and most of the community work within.

"It's a reflection of the areas that they work."

The union says it started its action after realising the employees were being underpaid by an average of $20 per week