It seems our argument is rather "chicken or egg" isn't it. Of course an early workaround would be a Britain more dedicated to convict transportation, OTL it was definitely mismanaged around lesser departments and outsourced to corrupt shipping magnates. This would get more people here without relying on personal and financial incentives inevitably drawing them back to the coastal cities. About 20% of the country draws descent from convicts today, and they were a relatively small base population of 160,000 in less than a century. If you even double those figures we'd achieve a population above 30 Million and have the added benefit of being able to localise them at your leisure. I understand that the transportation was ended due to the distate of free settlers, but when did the British Empire give a fuck what the colonies thought?
I've wondered about increased convict transportation before. I think that in practice there's a soft cap. While Britain could send more, the sooner there's a large enough number for a proper economy to develop there will in that economy be individuals with power and influence that really do not like the convict association. The British Empire evidently cared a fair bit, as the colonies OTL were able to stop transportation when it suited them, more or less. On its own, I don't think more convicts will have a huge impact. Perhaps a more radical departure from OTL would be plausible - for example, a Fijian style forced migration program would be very interesting. Hard to imagine a pre-1850 avenue for this though, given the transportation costs of plausible export goods that require lots of labour.
Population growth in Australia probably needs to be driven by free settlement, and the sooner this becomes self-sustaining the better. The longer it keeps at a high rate the better as well. It is very notable that the 1890's depression totally removed Australian access to the extraordinary migration rates of the 1890-1910 period, scooping up a tiny bit in 1910-1914. Avoiding the depression, or rather reducing its length, would help greatly as well.
I would suggest that if one were to right a TL with 'maximum Australian population' in mind, and didn't want to 'cheat' through screwing over large parts of the rest of the world, a range of plausible 19th C developments could include:
Successful 1803 Port Phillip settlement: Not a huge change initially but IMO could have a much larger economic impact by the 1830's and 40's given the opportunities of Victoria versus Tasmania for economic development/land ownership attracting more people during the initial population boom pre-gold rush.
Early South Australian Experiment: AFAIK there is no particular reason why the South Australian idea could not have occurred a decade or so earlier than OTL. Not a massive impact on its own, but potentially helpful in the long run.
Early Gold Rush: This one is massive, but really hard to engineer. Gold was found many times historically, but didn't kick off a rush till after California. The US did have gold rushes prior to this one, like Georgia, perhaps that could somehow lead to an early cultural shift in Aus? Or to go a more extreme route, I don't think it'd be too difficult for the Mexican-American War to break out in the late 1830's or so, with a similar result and say an 1840 California Gold Rush being followed in 1841 by the rushes in Vic/NSW. An extra 10 years of growth could make a huge impact, easily seeing a population of 2.5 million by 1870.
Early WA Gold Rush: This one is actually really easy from an AH point of view. Gold in Coolgardie/Kalgoorlie was basically waiting to be found by luck, there's no reason it could not be found in the 1860's or 70's. Combining this with the previous POD makes it even easier. This will have
huge implications for WA, basically creating another state capable of attracting migrants for non-gold reasons by the 1880's. WA's population could easily be 1-2 million bigger today with this POD.
Early/lessened Depression: This one was massively exacerbated by the influx of British money in the 1880's, turning a natural downturn into the most severe in our history. Had the banks blown themselves up in the 1870's instead, again perhaps at the tale end of a gold rush that started a decade earlier, and prior to international finance becoming quite as saturated as it would by the 1880's, it is plausible IMO that the depression would be mostly over by the early 1890's. This coincides with the 1890's migration boom, and is before the formal WAP. Still no non-Whites will be coming in in huge numbers, but perhaps if industry is also a little stronger a very large number of Eastern Europeans could come.
None of these are mutually exclusive, IMO, and could add up to a population of around 6-8 million in 1900, and 8-10 million in 1910. So basically a bit over double OTL, more or less. Such an early POD makes the 20th century pretty much impossible to guess, but could go in many directions.
My question is could this western coastal area here north from Perth approximate the population density on the eastern half of the continent?
I know the continent interior is very harsh but I've never fully understood why the west coast aside from Perth is so sparse and could it support more Perth sized cities?
There are two reasons why WA's population is a tenth of Eastern Australia's.
The primary is that while the South West corner is perfectly hospitable to Europeans, north of about Geraldton is very unpopular. There's no Queensland equivalent. Despite certain Westralian stereotypes, Eastern Australia is actually a much larger and richer piece of real estate, going a lot further north and south. The secondary reason is that population growth there basically began in the 1890's due to a late gold rush, which if earlier could indeed have seen a larger population out west.
Given this is pre 1900 I assume the regional towns would gain sufficient critical mass like Geelong, Ballarat and Bendigo did from the gold rush in the 1700s because there's likely no saving them in the 1900s.
Oh I don't know, if you want to go for a more exotic POD like a Cuban Missile War we could easily see mid 20th century regional Australia be inundated with millions of British refugees in need of a home!
'Exotic' PODS are those which IMO are basically non-Australian but which could have a major impact on Australia's population. A good one could be a Trent War which sees the US blockaded for several years, re-directing several hundred thousand migrants to Gold Rush Australia at a crucial moment in population growth. This could be followed by a much dampened enthusiasm for US migration in the 1860's and 70's, seeing a more permanent shift to Australia and other centres like NZ, SA, and Argentina. Spice it up further with a second war in the 1880's that sees the US win the rematch and a million or so Canadian/Confederate citizens look for somewhere new to live, with space and jobs... down under. Or maybe have the British lose WW1. Kinda cheating though IMO.