For instance Sydney is constructing the 'North West Rail Link' at present. An underground line similar to the 'north west rail link' in Sydney could be built to Doncaster and beyond. To service suburbs such as Doncaster East, Donvale, Park Orchards and Warrandyte. It is needed as Manningham is one of the only areas in metropolitan Melbourne without a train line, private car ownership is relatively high across the Eastern Region compared to the metropolitan Melbourne average. Which is a similar make-up to the North West region in Sydney.
This in itself is not a legititmate reason for building a railway, underground or not. The fact is, there is little growth in this area of Melbourne, this is why other areas are seen as having a higher priority.
While I do agree with you that more new infrastructure will be needed, I believe your argument is fundamentaly flawed as you are using simple comparisons with Sydney and suggesting "They have it, why don't we?".
If Melbourne was to do everything that Sydney does, than would we have pulled all of our tram network up? Would we have built a massive Monorail only to then pull it back down?
Sydney isn't perfect, neither is Melbourne, but the solutions to Melbourne's problems do not lie with a basic comparison to Sydney. They are within well thought out planning, similar to what we have seen with the plans for the Melbourne Metro Rail tunnel, something I'm hoping gets built in the near future.