You are right Raver
National Jet and Skywest have no height restriction. Tiger Airways have a minimum 158cm and no upper limit. Therefore I doubt it's a CASA rule. Unless it's a CASA requirement for Qantas' AOC - I find that unlikely though.
I would say Qantas is strict with it's limits because of its own OH&S policy.
You don't want to be banging your head on the overhead monitors on those CLASSIC 737-400s now, do you Raver!