As Physics said, its very hard to tell. The universities dont set or choose the ATAR before hand, its all about supply and demand. The only reason it stays roughly constant between years is because people have roughly the same wants and desires and preferences from year to year and the number of places usually stays constant as well, so, the 500 spots filled in a year will tend to be around a similar score as last.
Sometimes we have huge jumps, like here, which is massive. I remember the exact thing happened when i was applying for uni and they said they'd open up more places (so increase the supply) so the clearly in would drop, perhaps they're doing that again this year.