I believe that Melbourne airport originally planned on a parallel 16/34 but were advised by Airservices that this would create bottlenecks due to aircraft having to cross the current 16/34 to access the terminal areas.
A parallel 27L/09R seemed a perfect answer until they came up against the realities of noise sensitive areas, Essendon airport operations, loss of LAHSO, not being able to make either parallel 27/09 long enough for heavy long range departures, and so having to retain 16/34 for departures which would have to cross not one, but two, active 27/09 runways!
Hopefully they have re-thought the issue and will find the money to fill in the ravines to build 16R/34L and finance a taxiway that allows aircraft to taxi under the final approach path of 16L/34R as is the case in Frankfurt. In the meantime there is a perfectly good airport at Avalon to take airlines that do not need to interline with domestic flights.