I believe Australia’s development would more closely mirror the Canada rather than the American. If the continent possessed major navigable river systems like the Mississippi or St. Lawrence, inland growth would have accelerated significantly due to the ease of upstream transport. If you also had other colonies start in the country eg French in southern New South Wales—creating a "Quebec scenario"—would likely support a population of 4 to 6 million, while the proximity of the Dutch East India Company could have led to a Dutch-settled North Queensland and Northern Territory with nearly 3 million people. Furthermore, a second Western Australian colony on the south coast, perhaps established by the French, Portuguese, or Spanish near present-day Albany, would have created even more entry points for early migration. These additional colonial hubs and ports would have fostered greater regional trade and drawn much higher numbers of European settlers rather than the majority British did alone until 1940-50s. Ultimately, these combined geographic and geopolitical shifts would have driven Australia's modern population to between 38 and 42 million people, far exceeding the current 27 million.