The track shown on the ATSB map for VH-CGT indicates the acft may have been asked to report at the Moorabbin Oval, the origin of the track shown on that map. This is a frequent request by MB ATC for acft joining the circuit after reporting inbound at Brighton, one of the complusory approach points for Moorabbin Aerodrome.
Therefore the postition of CGT as shown would be fairly normal for joining downwind for RWY31L. The position of UPY would also have been most likely correct, but the problem seems to have been that of a climbing acft being seen against a background of housing etc. by the inbound PA28 crew in sufficient time to take more avoiding action than they did.
This is not withstanding the apparent lack of radio comms by UPY and the situational awareness by the inbound PA28 crew of all the relevant circuit traffic. The pilot of UPY with his limited experience may have been only concentrating on his own circuit and height to the exclusion of almost all other considerations.
My assumptions based on a bit of knowledge of Moorabbin procedures.
Blackburn