Foreign airlines were able to operate domestic sectors within Australia with their own passengers, these flights were not "generally available" to the public.
Eg back in the 1990s BA flew Sydney - Brisbane with passengers who were entering or leaving Australia with them. You could arrive in Sydney from London, stop over for a few days and then go on to Brisbane. Passengers got a special boarding pass and still had to go through customs.
With the relatively small market and large distances involved it may not have been worth having two services to different cities but by combining them into one it paid. Africa - Perth - Sydney would be a good example of this.
What Jetstar appear to be doing is something different. An Australian Airline with an Australian registered aircraft flying a domestic sector with seats generally available to the public. Assuming the crew enter Australia on the general declaration, entry would be permitted for the stated purpose only ie to transit and return home, not to engage in employment.
In order to work in Australia you need to either be a citizen or posess the required visa and this doesn't seem to be the case here.
An immigration compliance officer meeting a VH registered Jetstar aircraft in MEL, which has just arrived from DRW would probably detain foreign nationals found to be working on board as they would be in breach of the entry conditions given on arrival in Australia. The domestic sectors in this case should be flown by people with the right to work in Australia, do the foreign flight attendants have the right to work here ?
Could, for example Virgin Blue employ a few Thai Airways cabin crew to do domestic sectors for them during their stop overs ?
If the Jetstar service is flown with a Singapore aircraft and carries only passengers originating or terminating in Singapore then there should be no problem with the domestic service having a foreign crew.