Two pilots were taken to hospital for precautionary checks after making an emergency landing at Canberra Airport on Thursday morning.
Emergency services were called to the airport at 11.46pm on Wednesday after a sudden loss of pressure in a cargo plane.
The Australian Transport Safety Bureau said the crew donned oxygen masks and descended to 20,000 feet after receiving a "wing body overheat warning", but the first officer was incapacitated.
The captain then descended to 8000 feet and diverted the plane to Canberra.
Firefighters and paramedics were on standby at Canberra Airport when the plane is understood to have landed safely just after midnight.
The two pilots were taken to Canberra Hospital for monitoring, but were released later in the morning after precautionary checks.
Their aircraft, a Qantas Boeing 737-376 cargo plane, was en route from Brisbane to Melbourne when a technical fault with the onboard air conditioning system caused the pressure loss in the cabin.
The Australian Transport Safety Bureau is investigating.
It will collect evidence to find out exactly what caused the incident.
"As part of the investigation, the ATSB will collect and examine information on the aircraft’s flight data recorders and interview maintenance and flight crew," it said in a statement.
There were no passengers on board.