The Portuguese supposedly sighted Australia in 1522, but didn't stick around.
Dutch sailors sighted Australia in about 1607, were attacked by Aborigines and didn't stick around.
So the Portuguese colonize SE Australia, around OTL Victoria, and expand into Eastern and Southern Australia.
The Dutch colonize Cape York, and dominate Northern Australia.
The Dutch sailor Abel Tasman sets up a colony in Tasmania.
The Dutch encourage emigration for several reasons. To get rid of religious and political dissidents, to sell ships and supplies necessary for getting to Australia, and to exploit the vast wealth of the continent, and to take over the continent before the Portuguese do. (This is the period of those big religious wars in Europe.)
The Portuguese have many of the same attitudes, and encourage colonization.
The Aborigines are caught in the middle, and appeal to somebody from Indonesia for help. The Indonesians invade, "protecting" the Aborigines. I don't know how much "protection" the Aborigines would really get?
So the 3-way war ends in a tie, with the continent divided between Portugal, Holland and Indonesia, and the population is at least what it was in OTL 1800.
(There were lots of deaths, but lots of immigrant colonists.)
Did you want the continent to be one country? Maybe the Dutch throw out the other two powers, or the Indonesians or Portuguese do.
Because Australia is not available, Britain sends its convicts to North America. They hold onto Quebec, and make concessions to the Americans, so the American Revolution is avoided. Because Britain is concentrating on the Americas, and manages to get some of Latin America from the incompetent Spaniard rulers, they don't colonize the Pacific as much.
As a result, European colonists in Australia send out expeditions to New Zealand, Hawaii and other places. A famous Dutch-Australian artist settles in Tahiti and founds a colony there, for example. So the Pacific becomes an Australian region.
Militarism rises in Japan, they see Australia as a threat. Australia supports the Okinawans against Japan, and a war breaks out over control of Okinawa.
Japan invades China and Australian territories, and at the end of the war Japan is divided between China and Australia, with Sakhalin Island left for the Ainu to have their own republic.
The Russian Revolution happens around this time, and many of the Romanovs are would-be warlords who try to take over, or at least conquer little bits and pieces of Asia, and the Russian empire collapses.
Twenty years later a new puppet emperor gains the throne and the Australians let him sign a new treaty and new constitution, and withdraw from their part of Japan.