My understanding is you have an obligation to lodge a tax return with the IRS. Any income earnt in the US must be declared in both Australia and the US. There is a tax treaty between the 2 countries so that you don't pay tax twice.
The amount of capital gain is dependant on how long you have held the property and what structure you set up. If you buy a property in a c-corp you will effectively pay tax twice (I personally don't use this structure). If you elect to have an LLC acting as a pass through entity then the capital gains tax rate can be as low as 15% in the US. The right structure for you is dependant on your personal situation.
A lot will depend on how you structure your purchase in the US. Many Australians elect to purchase US property in an LLC structure. LLC's can be owned by individuals, trusts and even SMSF's. I would recommend seeking the services of a US CPA that is familiar with foreign clients and an Australian CPA that has experience dealing with US investing. I can recommend someone if you need help.
Good luck with your purchase. Make sure you check comparable sales on the property you are buying to ensure you are not overpaying. If the property is located in Atlanta Georgia I would be happy to look it up for you.
Also if you have not come across this website I suggest you check it out.
www.zillow.com
It provides historical information on properties by entering in an address.
If you are purchasing a turn key property "already rented" you may be paying an inflated price. I would encourage you to do some homework and understand the values of the surrounding homes before purchasing.
Disclaimer:I am not a tax professional and the above cannot be used as advise. Please refer to legal and tax professionals.