Why CASA? Surely ATSB are the folks who should be invited to be present?
Consider which NAA is the certificating authority ?
https://www.mahindraaerospace.com/ma...20Rev%2022.PDF
ATSB would have been invited to participate by the State of Registry.
I am only speculating, here, but it would not be unreasonable/unexpected for the ATSB to request either a Gippy Aero or CASA accredited representative to go on ATSB's behalf.
When I last was working with ASTA, I was part of a two-man team which went on ATSB's behalf to an O/S Nomad fatal - quite some years ago, now. The NTSB was a tad busy with other stuff at the time so we ended up running the bulk of the investigation, as it turned out - a bit unusual, but them's the breaks at times ..