It is unless it's integral to the job that you be a certain size or weight.
F/As have a minimum height so they can open the lockers etc, you could probably specify a maximum weight for helicopter pilots because that can impact on the payload, but otherwise you'd want a good reason ready. As you say, if someone has a medical and fits in your aircraft you probably wouldn't want to use the 'you too big' line.
It's nothing to do with HR practices and everything to do with the Australian state and federal anti-discrimination legislation, which is much more restrictive than what you get in Europe, or even the UK. There are also clauses about indirect discrimination, where it wasn't intended but occurred anyway. Virgin got hit with this one by a couple of rejected F/A candidates, where the company was found to have discriminated (on age), but unintentionally.