Really a question for AU border force, (not FT). Generally an immediate turn around requires full visa. (not transit)
Would expect SQ will deny you boarding as no entry to AU.
In Australia international arrivals and international departures are physical separated. Those doing an air side transit go through transit security/transit door.
While the aircraft will be expected to turn around that is not 110% guaranteed. While unlikely, aircraft can have a technical fault, weather delays and/or crew issues that can delay departure.
AU govt -->