I'm pretty sure the fares were set by the government
They were, by an appointed Commission, initially fares were identical, then allowance for fare differences were permitted, but only that permitted by the Commission. I've read that Ansett and TAA had to depart five minutes apart with Ansett going first. Not going to post a link in order to give the Mods an easy time but a search for "THE 1981 AIRLINES AGREEMENT", by H BURMESTER is informative.