Nothing wrong with the Garmin 155XL. I think it has the same GPS engine as the Garmin GNS430 (not 430W). Unless the aircraft has recently had its ADSB upgrade, it looks like its coupled with an Avidyne MFD in this aircraft.
There are plenty of jets still using the 155XL.
The pilot (assuming its the normal pilot for that aircraft) is very competent and has been flying this aircraft for 10 years or more. You've got to assume that 90% of us would have had the same issue in the same circumstances.
This incident shares with the old Benalla incident that radar data on the aircraft's flight path, but Airservices Australia did not provide any advice. In a first world country we should expect better than this.
Before the ATC guys jump in and complain, these comments are directed at the system, not the controllers.
The other issue it raises is the adequacy of the "backup" network of ground based navaids that we are going to be left with. If the Mt Livingstone NDB and / or VOR was still operating or the Eildon Weir VOR, then the pilot would have had a cross check. But the system that AsA are unilaterally leaving us with does not provide this.
After the criticisms of the ATSB over the diligence of the investigation of the Benalla incident and the Lockhart river incident. Plus the criticisms of the collusion between CASA & the ATSB over the Pelair incident, this will be a very interesting report.