It's certainly clear that what J* are doing here is different to what occasionally used to occur on the QF long haul fleets. We used to regularly fly QF10 SIN-MEL-SYD and clear customs in Sydney. It's important that J* management isn't able to imply that this is what they're doing when they're flying JQ62 SIN-DRW and then changing to JQ 'whatever' from DRW to [insert other Australian city here]. They're not 'continuing the service' to another city and then clearing customs in that city. They're operating a domestic service.
The exception would be if they still do the QF62 SIN-DRW-CNS. That they can have if it's the same flight number and operated as an international service with crew clearing customs in CNS.