If you're carrying anything larger than a daypack, do not even think about using the lockers at South Bank, which are both few in number and absurdly small - as I found to my huge annoyance last week (January 27, 2016). It is pretty clear the lockers are solely intended for the use of swimmers at nearby Streets Beach, as is also suggested by the maximum hiring period of four hours ($4).
(NB: Be aware that there is no backspacing when registering your PIN - so forget any mistyped number at your peril.)
The only alternative is to leave your luggage at the airport then spend $35 and more than an hour returning there to retrieve it. Yes, utterly ridiculous and a petty, provincial-minded over-reaction to perceived terrorism threats.
Surely there is scope for a company like Smarte Carte to operate a supervised storage facility somewhere like Roma Street Station or at Queen Street bus interchange.