For Capital Gains Tax (CGT) purposes, your home qualifies for the main residence exemption from the time you acquire it, provided you move in as soon as practicable. 

If you acquire a new home before you dispose of your old one, you can treat both properties as your main residence for up to 6 months under certain conditions.

You can claim this exemption if all of the following are true:

1. You lived in your old home as your main residence for a continuous period of at least 3 months in the 12 months before you disposed of it.
2. You did not use your old home to produce income (such as rent) during any part of that 12 months when it was not your main residence.
3. The new property becomes your main residence.

If it takes longer than 6 months to dispose of your old home, the main residence exemption applies to both homes only for the last 6 months before you dispose of your old home. For the period before this, when you owned both homes, you can choose which home to treat as your main residence. The other property will be subject to CGT for that period.

Reference :

ATO – Moving to a new main residence