"If you sell a capital asset, such as real estate or shares, you...

"If you sell a capital asset, such as real estate or shares, you usually make a capital gain or a capital loss. This is the difference between what it cost you to acquire the asset and what you receive when you dispose of it.

The point at which you make a capital gain or loss is usually when you enter into the contract for disposal, not when you settle. So if you sign a contract to sell an investment property in June 2017, and settle in August 2017, you need to report the capital gain or loss in your 2016–17 tax return."

https://www.ato.gov.au/general/capital-gains-tax/