Industry and Investment New South Wales said up to 11 adult males were found in traps at Hillston, in an existing fruit fly exclusion zone.
Satendra Kumar, the NSW director of plant biosecurity, told the ABC the discovery was worrying and would be costly to growers.
"If there are no outbreaks, it's an area which is free of the fruit fly. Therefore fruit can go into sensitive markets without treatment," he said.
"Now we have a situation where we have an outbreak, therefore any fruit moving out of there needs to be treated before it can got to a fruit fly sensitive market."
