The 6 year period can apply to a residence that has been rented out if you own another property. You are then choosing to apply the main residence exemption to the rented property for a maximum of 6 years rather than the second property. The second property will then be subject to CGT when sold in relation to the period it was not treated as the main residence.

It says on the ATO website 'where a dwelling that was your main residence does not stop being your main residence and you use it to produce income, you will only receive a partial main residence exemption.'

I'm on mobile at the moment and can't figure out how to link but if you google 'treating dwelling as your main residence after you move out ato' you will see where the above quote comes from.

I am an accountant so have dealt with this a bit but there are a multitude of varying scenarios that can occur in these situations which is why we don't provide advice and just tell everyone to talk to their accountants. But thanks for the downvotes.