Melbourne's expected to be home for 8 million people by 2050. This is a problem considering how overpopulation is already a challenge there now. The coalition government's suggesting decentralizing Melbourne by encouraging people to move into the neighboring regions. A high speed train system connecting the regions to the city is part of the proposition, so it makes sense for those living outside.
The opposition however means to expand Melbourne itself by integrating the necessary infrastructure including an inner city loop that would cost 50 billion dollars and 30 years to complete.