The vaccine can never be mandatory for Australians to enter Australia. Australia *must* accept its citizens if they arrive at the border. (The PM has also said the vaccine will not be mandatory for people living in Australia.)
So assuming an Australian can get to the border, they will be let in. If they choose to go into quarantine rather than have the vaccination, that's perfectly fine
Your example does however highlight one of the issues around airlines saying they won't fly someone unless they've been vaccinated. While non-citizens may require vaccination to enter Australia, and the airline can refuse to carry those non-citizens in the event they aren't vaccinated, the same can't be said for citizens. So I'm not even sure Qantas would be able to write that into their terms and conditions.
While covid may not kill young people, they can certainly carry and pass it on. have you considered how you would feel if you passed covid on to your parents and they became seriously ill?