Dizziness, breathlessness and a warming feeling in the chest were the first signs that something had gone seriously wrong on the short flight from Moree to Tamworth.

A visual scan revealed the carbon monoxide card detector was gradually getting darker, so the pilot opened all the fresh air vents and the storm window before returning the plane to Moree.

Large exposures to carbon monoxide can lead to a loss of consciousness, heart arrhythmias, seizures and even death.

At the time, the pilot was just 19 kilometres south-east of Moree airport and a medical examination after the flight revealed elevated levels of carbon monoxide in the blood.

The incident in the Piper PA-28, VH-TBB has been classified as "serious" and is under investigation by the Australian Transport Safety Bureau (ATSB).

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It took place on September 23 about 2.18pm and the pilot escaped without lasting injuries.

The pilot will be interviewed by investigators and a review of aircraft maintenance information will be taken.

If a critical safety issue is identified during the investigation the ATSB will immediately notify the parties involves so safety actions can be taken.

A final report will be released at the conclusion of the investigation, expected to be finished in mid-2021.