Jabba has correctly identified it - unless there is an across the board direction for all domestic airlines to comply with a fixed carry on limit then it will be a case of each airline waiting for their competition to impose the draconian rule.
The pax mantra is check in nothing, carry on everything, get on early and claim the locker space. If seated in economy, put your bag in biz class locker as you walk through.
While wating for my flight yesterday I watched a CC boarding pax in Syd get 'overuled' and the passenger then carried on a largish wheelie and two other duffle type bags.
It is first and foremost a safety issue and I really wonder how a 737-400 can be safe with just one CC down the back during boarding and all those pax just leaving the bags in aisle for the CC to sort out when there is no overhead space.
Actually for the CASA watchers: - how many covert audits have been done in connection with this issue? (observed from the last row)
AT