[ssiguy]

Sydney & Melbourne are not akin to Sao Paulo & Rio or Toronto & Montreal because in the last 2 examples, you had the historic larger city overtaken by the smaller one and the difference has only increased in time to the point where the discussion about which are the country's dominate city has long since been decided.

Sydney & Melbourne, however, are similar sized cities but neither is growing leaps and bounds over the other. Melbourne may overtake Sydney now but in just 10 years, Sydney could once again become Australia's biggest city and 10 years after that Melbourne could regain it's title. The two will be having this conversation for decades or more and the question may never be settled.

A more apt comparison is Dallas/Fort Worth & Houston. Like Syd/Mel neither is a capitol city and yet both are growing quickly and which is bigger often comes down to where the boundaries are determined but neither will ever become dominant over the other.

In terms of how the rest of the world views the importance of the 2, Sydney will probably always come out on top, Not only is it the primary air entry point to the country {which means a lot on an island nation} but also because it is so recognizable with it's famous Sydney Opera House and Harbour Bridge. Melbourne has no such icon that is automatically associated with the city. Yes, Melbourne has a fantastic skyline but if you were to ask 20 different people outside of Australia to name the skyline, you would get 20 different answers which is certainly not the case with Sydney.