People, my two cents worth in what I think happened at BLA. Aircraft on this route almost every week. YSBK to YBLA. DCT takes you over AY and WGT aids to line up on BLAED the northernmost start point for the Benalla NPA. What we know, a diversion down to Ulladulla cleared DCT BLA from around that position. Numerous alarms in ATC to show aircraft is off course. Witness statements that an aircraft was heard overflying Myrtleford heading SW in very heavy cloud. The crash site just to the south of Mt Bellevue at a height of some 1700ft. Radar data has been said to be very unreliable in that area.
My opinion in what has happened. Friends of family on board, including a very capable helicopter aviator. Two pilots up front one regular, one not. One 'not' may have input the data into the trimble. Could a "BLAEG" be mistaken for a "BLAED" or "BLAEE"? Starting from a non normal position a DCT is given from position to BLA. No aids on track to verify. If BLAEG is input then the cheese lines up with the reported sighting near Myrtlford Ulladulla and the crash site. A straight line! The altitude at the crash site is way below the NPA until way inside the final approach fix. I believe this may have been the case due to the pilots believing they were heading for BLAEE and expecting the command to turn right to head for the FAF. The track from "I" to "F" is over low ground, an assumption could be that pilot having flown this trip hundreds of times before knew that ground clearance allowed you to be lower than profile. If pilot was challenged by ATC to ask if ops normal I am sure the pilot would have replied in affirmative. UNLESS the specific question was "Confirm tracking for GOLF" ATC now have these waypoints showing on their screens where before they were not. The information was not available to ATC to even warn pilot.
A terrible tragedy. As always, the disaster averted at any time if the pilot was alerted to his error. I fear the pilot "knew" where he was, a shame it was in the wrong spot.
GPS is a fantastic device, just make tripple sure you are being taken to where you want to go.