Jetstar enforce the most heavily (at the gate) however it is still random. They also typically weigh and tag carry-on at check-in too. There is an option to buy additional carry-on allowance ahead of time (up to 10kg I think).

I have seen Qantas staff intermittently weighing bags at security queue in both Sydney T3 and Melbourne T1, however I'm unsure what (if any) action is taken if you're overweight. I've never experienced any enforcement from Qantas about carry-on and the sheer volume of bags you see people bringing onboard suggests it isn't common to be pulled up over it.