One possible answer is for ATC to say "XXX I see you at/passing xx altitude, turn left...." which would prove the call came rom ATC.
Big difference here is Avalon was involved. A CTAF airport for half the day, Jetstar has 4 movements within CTAF frequency (most during darkness). Avalon is pretty busy after hours with the flight schools in Ballarat and Moorabbin using it for circuit training so a few odd calls wouldn't probably raise alarms.....
Fake calls, bad weather, late night, CTAF.......yeah no thanks. Hope they find them.