A Qantas captain and co-pilot were left temporarily “incapacitated” and “gasping for air” during a terrifying mayday incident three years ago, a new aviation safety report has revealed.

The Australian Transport Safety Bureau report released on Thursday shed disturbing details about an Express Freighters Australia aircraft, which was travelling from Brisbane to Melbourne on August 15, 2018.

The crew donned oxygen masks when a caution light flickered, and the aircraft started an emergency descent.

But during the initial part of the descent, the captain of the Qantas Boeing 737-376 was left “temporarily incapacitated” and “gagging and gasping for breath” by a reaction to the increased supply of breathing oxygen from the mask, according to the report.

A mayday was then declared by the co-pilot, also known as the first officer, and the aircraft started a diversion to Canberra Airport.

“The first officer, cognisant that they had previously discussed feeling unwell, observed the captain slumped forward, gagging and gasping for air,” the ATSB said.

“The first officer recalled checking on the captain during this episode but not receiving a response.”

After the captain recovered, the first officer then suffered “incapacitating symptoms consistent with hyperventilation”.

The captain radio called air traffic control, requesting the attendance of emergency services upon arrival at Canberra where the aircraft landed safely.

The report noted that at “no point” were both pilots incapacitated at the same time.

The investigation identified intermittent flickering of the master caution light and overheat annunciator were likely due to an electrical fault in the right wing-body overheat detection system.

“During the descent, the captain selected emergency flow on the oxygen mask resulting in an ingestion of gaseous oxygen, causing their temporary incapacitation,” the report read.

“The operating flight crew conducted the appropriate non-normal checklist, however the overheat indication could not be rectified due to the fault in the wing-body overheat detection system,” the report read.

“An additional fault with an isolation valve in the aircraft pressurisation system prevented isolation of the right wing-body pressure duct.

“This led the crew to conduct further troubleshooting during which the cabin air supply was reduced.

“In conjunction with a higher than normal cabin leak rate, the reduced airflow also lessened the cabin pressure.”

After landing, both the captain and first officer were transported to hospital via ambulance for medical assessment.

Post‑occurrence medical testing and assessments did not identify lasting effects from the flight.

During inspections after the incident, Qantas engineers identified “a range of serviceability issues” with the aircraft fuselage cabin drain valves, fuselage door seal, and the auxiliary power unit duct bellow seal that affected the capacity for the aircraft to hold cabin pressure.

According to the report, Qantas advised ATSB it incorporated maintenance changes to avoid similar incidents in the future.

In a statement to NCA NewsWire a Qantas spokesperson said: “After this incident occurred back in 2018, we updated our policy on how pilots troubleshoot technical issues during flights to avoid these actions inadvertently causing other consequences as occurred in this incident.

“While these freighter aircraft were maintained in accordance with the manufacturers’ requirements, we have since put in place additional checks to the maintenance program.”