Welcome.
This post sounds strangely like your asking for locations which I think you'll find is frowned upon and very unlikely to get any responses.
My suggestion to you is to post some stories of your own explores with pics and things and then maybe when people get to know you they'll be more inclined to throw you a few location tips.
I too am from Melbourne and would be more than happy to do some expos with you sometime if you'd like but as I said you won't find many of us willing to just drop locations to 'noobies'.
Having said that I have heard tale of some tunnels below the RMH that you can apparently access via ladders from the RMH basement but I'm not sure how accurate this info is or how hard that even is to access or where said tunnels even go.
Wander often. Wonder always.