depends on what course?
some courses have no ATAR cut off. e.g. medicine, and courses u need to audition for, so, dance, music, and there some where u have to fill in additional forms, so of course for those it'll be based on UMAT+interview, your audition, and extra stuff you needed to apply.
not sure which course you're talking about though.
Depends purely on ATAR rank. First in, best dressed, until there are no more places left.
So for example, UNSW has the BE ME (Electrical) which is a new course this year. They recommend 96+ but there is no set cut off because cut off is based on supply and demand, and how many places they can give out each year. So the first say 100 students by descending ATAR will get the offer.
Placing it first has its advantages, so long as you have a decent ATAR, but it depends on how many people have an ATAR above you who haven't been offered a course yet.