That isn't quite how it worked.
Princess originally planned to schedule a ship each for Brisbane & Melbourne, Crown Princess plus two summer ships in Sydney. In the end they split 24/25 season for Diamond between both home ports, and also removed a ship from Sydney. Of course Melbourne should sell out - what else do you have apart from a short season of NCL and P&O. Brisbane also has Royal, Encounter & Luminosa to compete with.
Yes, a dummy spit with the Vic government, but with context, the same govt that waived most of VV's port fees that was insisting on all of the Carnival Corp brands copping an increase. Some would call that standing on principle, so some of the angst could be redirected to your government & port authorities. Grand may have been sailing mostly full, but they still weren't turning a profit. AHPPC held the cruise ban, and there was nothing our pollies could do about it.
In the end, 25/26 both ports miss out. Not just the cities, but we all miss out on Melbourne & Brisbane departures, so it doesn't really serve them right if we are the ones that are missing out. Despite the recent spin of Aussie cruising breaking records, things are on the slide now that covid FCCs have been used-up. Less home based ships in all of our ports over the next couple of years, but hopefully our govts will let us all have a bit more pocket money in the years that follow.