During a routine inspection, a deputy made the measurement at an install road behind a holing stopping.
The stopping was built to segregate the two districts before holing with the continuous miner on the face road.
The NSW Resources Regulator said a venturi was set up in the stub to maintain ventilation, however, the air hose had been removed.
"Workers must not interfere with ventilation devices such as venturis, fans, regulators and stoppings without a thorough understanding of the impacts of the ventilation system," it said.
"Compressed air lines supplying ventilation devices should be marked with an information tag to inform workers to prevent use of the hoses for other tasks."