Cut-offs are determined after the offers have been released. It is a supply/demand basis. You can't achieve the cut-off when you don't even know what it will be.
Generally, the cut-off is determined by the ATAR of the student who received the last offer. However, the published cut-off can be nominated.
So, if a uni has 500 spots available, you compete for 1 of the 500 spots, i.e. the top 500 ATARs will be accepted. If you are outside of the 500 spots, then you won't receive an offer.