Brisbane is the largest city in Australia in terms of population.  That is, when you take into account only the area governed by the city council.  When we talk about capital city populations we really shouldn't use the word city because that becomes confusing.  Really what you're talking about when you say Melbourne's population is 3.6 mill (or whatever it is now) you're talking about a metropolis (not a city).  City of Melbourne is tiny.  Geographically the city of Kalgoorlie-Boulder (in WA) is the largest city on earth. List of cities by size(area).  in any case, Melbourne doesn't come close to being anything even remotely close to the largest city in Australia in terms of the area it occupies.