About 4.30pm on Thursday, emergency services were called to a property on Thornton Road at Cathundral, 112 kilometres west of Dubbo, after reports a man and woman were unconscious in a water tank storing fertiliser.

Police have been told a woman entered the tank to assist a man, when she collapsed soon after entering.

The woman was treated at the scene by paramedics and taken to Trangie Hospital where she was airlifted to Orange Hospital for further treatment.

She is reported to be in a stable condition. The man was declared deceased at the scene.

Detectives attached to Orana Mid-Western Police District attended and have notified SafeWork NSW.

A report will be prepared for the coroner.