From an
article I wrote.
Capital Gains
A special rule applies when you move out if a property you purchased after the 20/8/1996 and commence to use it as a rental. The cost base of the property is reset to the market value of the property at the time you move out (s 118-196) so you will need to get a real estate agent to give you the value of the property at that time should you ever sell it. Please note that this reset only occurs once during your time of owning the property, and it occurs at the time the property is first used to produce income (ie first rented out). If the house was purchased before this date, the cost base is still what it was previously but the taxable gain is apportioned by the days it was held privately vs rented out.
Another interesting rule is the 6 year leave of absence rule. One you leave the property, you can choose to have it remain your place of residence (making the house tax free on sale) for another six years if you continue to rent it out, or indefinitely should you just let it sit. The only proviso is that another house cannot be considered to be your place of residence during this time. You do not need to make a final decision on which house will be your place of residence until you sell one of them. You can move back within the 6 years, make the house your place of residence again, move out and a brand new 6 year leave of absence rule can apply.
This rule is useful for people who move out to rent or move back home to their parents as they are not living anywhere that they own so they can keep the extension going. It is also useful for people going overseas and intend to rent out their house while they are away.
Also note that should capital gains apply to selling your house, if the house was purchased after 20/8/1991, you can add to the cost base of the property expenses you have not claimed as a deduction, even expenses that were incurred when the house was used privately. You cannot use costs incurred to increase the cost base if the costs were incurred before the house was reset to market value.
Of course capital gains does not apply to properties purchased before 20/9/1985 unless they are substantially renovated.
A typical misconception held by many investors is that if you move into a rental prior to selling it, it will be tax free on sale. This is incorrect. Once a house is not covered by a principal place of residence exemption, there will always be a capital gains tax liability if the property is sold.