Disco Stu,
Given the demise of FAC and subsequent privatisation of airfields accross the country my guess would be that Commonweatlth legislation notwithstanding, the state is within it's rights to introduce legislation that does not conflict. [does that make sense?]
Whilst the Victorian govt perhaps does not manage airports as such. The states seem to have a fair bit of influence in aviation matters at at state govt level. In WA that portfolio belongs to the dept of Planning and Infrastructure. Probably same horse different colour in Victoria.
I have looked through CARs and cannot find anything in relation to aircraft owners responsibility with regard to landing fees. It is perhaps for this reason that the states are taking their own initiative to rectify this anomoly.