Hello there,
In the US, individual taxpayers are generally required to file on a calendar year basis.
Corporations/businesses may file tax returns based on a short tax year basis depending on circumstances such as mergers/acquisitions, dissolution of partnerships etc.
In your case, it depends on your residence status. If you had become US citizens or green card holders, you would need to file a full year tax return for 2021. You can then explore the possibility to either exclude some of the foreign-earned income from Australia, or to claim the credit for foreign taxes paid to Australia.
https://www.irs.gov/individuals/international-taxpayers/choosing-the-foreign-earned-income-exclusion
https://www.irs.gov/individuals/international-taxpayers/foreign-tax-credit
If you had not become a US citizens or green card holders, then you would be treated as dual-status aliens. You would be taxed in accordance with the rules of taxation of Dual status Aliens (see link below).
https://www.irs.gov/individuals/international-taxpayers/taxation-of-dual-status-aliens
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