Faulty explained it really well.
This comes from a fundamental flaw with how POPs work. You cannot create POPs from nothing, you cannot change POP culture through event, and POPs will not assimilate to non-majority accepted POPs. Meaning, it's impossible to create a culture and make it prosper if it didn't exist at start.
An Australian culture would need to either exist at start or be "transferred" from another province. The former results in a more extreme version of what happens between Yankee, Dixie, and immigrant POPs in the United States where a Yankee plurality appears in a Dixie province and the becomes majority because all POPs start assimilating to Yankee since it's the primary culture. You would have Britons move to Australia, become plurality, and then cause all other immigrants to assimilate to British rather than Australian. The latter would mean that Australian POPs would need to be "stored" in some uninhabited province and then have an event/decision that kills the British POPs and then replace them with the Australian POPs when you feel that it's time for the Australian culture to appear, ideally when Australia becomes independent to prevent the assimilation issue. However, this not only requires the POPs to be stored somewhere like some random Pacific island or the Sahara Desert but is also unable to account for how big the POP size, ideology, etc. It's messy to say the least and usually doesn't work.
Simply put, it's just not feasible without changes to the engine that I'm sure Paradox just didn't feel like making.