Population of Tasmania = 515,000 and very decentralised (only ~40% liver within commuting distance of Hobart).
Population of Perth = 2,042,0000. Population of the rest of WA is 547,000.
Population of Melbourne = 4,936,000. Population of the rest of Victoria is 1,423,000.
Those city figures are for the "greater" city so that's anywhere that anyone's going to commute from, it's not simply the technical definition of the city proper.
So yes, these are all very convincing arguments for why Melbourne/Victoria is tremendously more challenging than the other places.
Tasmania is clearly very different as is regional WA but Perth and Melbourne aren't drastically different.
Not drastically different??? Melbourne is literally more than double the size! I'm about 90kg. If you put me in a fight with a 45 year old middle school student, are we significantly different?
The challenges associated with viruses become geometrically (ie, literally exponentially) more difficult with increased size, and you're talking about well over double the size! Plus, Melbourne has many towns and cities all around it, while Perth doesn't have anything like the same situation. Additionally, Perth has a population density of 295 people/km², while melbourne has over 500! Again, the difference in difficulty varies geometrically with population density and Melbourne's density is dramatically higher. Compound these two factors together, plus add in the surrounding town structure, plus Melbourne's demographic differences, and you're looking at something orders of magnitude more difficult in Melbourne.
One's larger than the other but only a bit over double, it's not several orders of magnitude or anything like that.
That one single factor is a lot more than double (50% more!) and the challenges don't increase arithmetically, they increase geometrically, meaning that yes, the challenge is orders of magnitude greater, especially when you factor in all the other issues of Melbourne.
It's akin to comparing a 737 with a 747. They're both large aircraft, we're not comparing the 747 to a single engine Cessna or a helicopter.
The comparison literally is much more like a single engine Cessna and a 737 than a 737 to a 747.
There may well be reasons why it can't be done but conceptually, well if something has been done apparently quite easily in a city of 2 million then it seems somewhat pessimistic to assume it can't be done at all in a city of 5 million.
If a realistic analysis gives a negative answer, it's just realistic, not pessimistic.
If it can't in fact be done well then that does raise one plausible long term outcome of this. Cities exceeding a certain size may well be simply too large and too much of a risk for this or indeed any country to have. If they're prone to uncontrollable pandemics then that's not an unreasonable conclusion, especially in a world where due to technology there's less need to have huge numbers of people in the one place anyway.
Population size is only one factor. Density, hygiene standards, climate, religion and culture, ethnicity, building materials, government type, geography, local wildlife and many others play important roles, and which pathogen spreads more easily varies differently with each variable. For some examples, this one seems to like cold climates, but others prefer warm climates. This one doesn't seem to use animals as vectors, but many other pathogens do. The amount of physical contact required to spread a disease changes population size and density importance (eg if it's something like HIV it doesn't make anywhere near as much difference, but if it's something like this virus it's one of the most important things, and population size and density are among the absolute most important).
I doubt we're going to see a limit on city sizes for this reason, but it's conceivable at some point in the distant future.