I am a student pilot based at Bankstown YSBK.
From what I have heard a Tobago was on left base leg to 29L or 29C (not sure which) for a touch and go, whilst a piper warrior (which later crashed fatally with 4 occupants) was on right base to 29R or 29C when they connected.
One witness reported the Tobago as making a sudden swerve to the right, where it clipped the rudder of the warrior. The warrior was seen to roll and crash, whilst the tobago (with instructer and student landed safely). The tobago pilots have not been charged, though they are under suspicion.
My "guess" is that the violent swerve to the right by the tobago sounds suspiciously like an evasive manouver, in an attempt to avoid hitting the warrior, which most likely strayed to the left of its intended path. I say this because the tobago had an instructer seated in the right seat who would have had a clear view of the warrior. My money is on the instructer. The pilot of the warrior was an experienced private pilot.
This is the only fatal that I have heard of at YSBK. There was one nearby at Hoxton Park a few years back, caused by both private pilots not keeping a good look-out for traffic.
Regards
HK