From www.bantacs.com.au: A clearance certificate is now mandatory for selling any real estate in Australia to prevent withholding 15% of the sale price by the ATO if not provided.

Only Australian residents for tax purposes are eligible for this certificate, and those who have moved overseas may not qualify despite selling their former home.

If the withholding occurs, sellers can claim a refund by filing an Australian tax return that declares the capital gain on the property.