Just to clarify, a metro means very frequent and very high capacity rail service that serves mostly dense areas in a city. Just very frequent or as politic call it 'turn of and go' system. Also high capacity, so other terms bigger trains. Misconception is it has to be single deckers since double decker can't run as frequent, this is not true. Also doesn't mean its all subways or means it a very fast service either.
The problem is that not all Australian cities aren't ready for a fully functioning metro system yet. It's because how our cities are dense in the core but the outer surburbs are sprawls of single housing. Metro can would work on our very busy lines which is what is proposed at Melbourne and Sydney. For Brisbane and Perth not quite yet ready but can aim for it.
My opinion is not use the word metro because it is just basically not a easy term to understand in Australia because we have no examples of this. Instead should call it "mass transit system" or something along the line
PS. Found a good definition at wikipedia, I'm surprised how accurate it was