If you earn over US$10,000 (or $400 for self-employed individuals), you are required to file form 1040. While taxes owed are still due by April 15th, expats get an automatic filing extension until June 15th. This can be extended still further online until October 15th.
If you have foreign assets worth over US$200,000 (per person), excluding your home if it is owned in your own name, you must also file a form 8938 declaring them.
“A non-resident company is taxed on its Australian source income at the same rate as a resident company.” The Australian Trade and Investment Commission.
If you had more than US$10,000 in total in one or more foreign bank accounts at any time during the tax year, you are also required to file FinCEN form 114, otherwise known as an FBAR (Foreign Bank Account Report).
If you are paying income tax in Australia, there are several mechanisms that allow you to avoid paying tax on the same income to the IRS too. The two primary ones are the Foreign Earned Income Exclusion, which lets you exclude the first around US$100,000 of foreign earned income from US tax if you can demonstrate that you are an Australian resident, and the Foreign Tax Credit, which gives you a dollar tax credit for every dollar of tax you’ve already paid in Australia. The Foreign Tax Credit is typically a better option if you pay more tax in Australia than you would to the US, as you can carry the excess credits forward for future use. Remember though, even if you don’t owe any tax in the US, if your income is above US$10,000 (or $400 for self-employed individuals) you still have to file.
The US and Australian governments (US Tax Accountant) share taxpayer info, while Australian banks pass on US account holders’ account info to the IRS, so it’s not worth considering hiding or being economical with the truth. The penalties for tax evasion for expats are tough to say the least.
If you’re a US citizen, green card holder, or dual citizen and you have lived in Australia for a while but didn’t know you needed to file a US tax return, don’t worry: there’s a program called the IRS Streamlined Procedure that allows you to catch up on your filing without facing any penalties. It’s better not to put this off though, lest the IRS finds you first.