Investigation number:
AO-2017-118
1.
"From this, the ATSB considers the levels of CO detected were likely to have adversely affected the pilot’s ability to control the aircraft during the flight."
2.
"From the above activities, the ATSB found pre‑existing cracking of the engine exhaust collector-ring, which could lead to exhaust leakage into the engine bay."
3.
"Further, the ATSB found a breach in the firewall from missing bolts used to secure magneto access panels in the firewall under the instrument panel in the cabin."
(my bold text)
It seems a pretty much open and shut case?
Evidence of the gas leak provided by Australian Transport Safety Bureau. Credit: Australian Transport Safety Bureau
(From 7news report)