It is different than you described in QLD ... As announced by QLD government, returning Australians arriving into QLD , will PAY their own expenses if they booked AFTER midnight on 17 June 2020... IF they booked their travel before then the Qld goverment will continue to pay the quarantine expenses. Their annoucment to that effect, on 17 June, is clear and precise.As best I understand, the question of paying for quarantine upon return is linked to the approval of outbound travel (at least, it was for my mate). Regardless of port of entry, he (his employer) had to pay quarantine costs. He quarantined in SYD and had to pay.
As I understand it, the "quarantine for free" only applies to returned travellers (as in those who have been OS since prior to the border closures), not those first leaving Australia. YMMV, but please share if your experience is different.