With a POD of 1900, make Austalia as large, powerful and prosperous as possible, without losing the national charachter of OTL Australia.
You need even more immgration from the UK,and Australia needs to open its door fully to eastern and southern Europeans like they did after WW2 but 40odd years earlier.
More immgration from Asian countries later in the century would help.
Maybe a British collapse in WW2 could force some sort of union with New Zealand along with aflood of British refugees would help.
Australia makes Papua New Guinea a state instead of giving it independence,and more assertive Australia can annex East Timour as well.
If all of these these thing happen you could have an Australia which includes most of the South Pacific islands with a population of 50million plus, economy and military at least equal to the UK or France.
1904 - Russo-Japanese war breaks out with Britain as Japanese ally. Russia declares war on UK.
What would make Russia do something so foolish? The parametres Anglo-Japanese alliance meant that Britain couldn't get invovled in the Russo-Japanese war because the alliance could only be initated if either side was at war with more than 1 great power. Tsar Nick' wasn't that mad. He expected he could beat 'backwards' Japan, but he knew he wouldn't be able to defeat Britain as well as Japan.
Unify Australia and New Zealand, give them the whites who leave India and the other colonies, have the claims for New Zealand, Fance, Australia, and the completly unclaimed chunk in Antartica be fused into one, get desalination plants around Darwin to start sending this inland, and short of annexing Samoa and the Solomon Islands, thats about it. Australians were simply afraid of people who werent white, even if their skin was more suited for the climate. I dont think Australians have more than a minor vestige of racism left though. Come to think of it, I have a way to maybe wank Australia. Keep Japan in the game. Make Australia be more built up. Though then they couldnt invest as much in industry for consumers.
No money there.
Australia can annex East Timour as well.
You don't think the Portuguese who own it might have something to say about it? Considering they were the oldest allied nation to the UK, I rather think the UK might veto the idea, don't you?
They wait until the Portuguese dump all their colonies.
One big thing: tHe biggest obsticle in Austalia is the Outback... is there a way all that desert and shrubland could be terraformed into something less... dead?
As for desalinization, would it be feasible long term and when is the earliest they could start doing that?
As long as the other states and territories adopt 3'6", I don't see an issue.Make all the colonies/states adopt the one, universal gauge for the railways.