SQ are continuing flights to Australia so there is a way for Aussies to get home, and Singapore needs to maintain its links with the outside world as it’s not self sufficient in many areas such as food production.
With QF not being a hub airline, it’s unable to concentrate pax and cargo loads from multiple destinations and would have to operate virtually empty direct flights which would be hugely expensive. Better to let another country bear the cost. The postal links are maintained, important cargo can still be delivered and the handful of pax who absolutely must travel are accommodated.
SIA crew are subject to very strict precautions including full PPE, confinement to their hotel rooms on layovers, wearing an electronic tracker and a COVID test on their return followed by more testing at regular intervals.
Vaccinations are already in progress for Singapore aircrews.
The right of return to your own country is a basic human right and Australia should be obliged to accept any citizen who wants to enter even if it means emergency quarantine facilities run by the army need to be provided.
This is a link to Wikipedia so it can’t be taken as definite however the article has links to official websites.
https://en.m.wikipedia.org/wiki/Right_of_return