Yeah! Remember the ATAR is just a rank, because it includes all students (including those who have dropped out or are ineligible for an ATAR) the median is closer to 70-something. With ~600 schools in the state, at a ~280 ranked school around the middle in (as a whole) average to decent scaling subjects it stands to reason that you would likely get in the 70s.
Don't get complacent that it's above your aim though, that estimate relies on the assumption that you will perform similarly (relatively, so actually better since everyone picks up their game at least a little for HSC) in the HSC to how you did in the internals. Not long left - may as well put in the effort.