Valuation and CGT

How does CGT and valuation work?
I'm considering selling an apartment in Sydney.
It was off the plan with initial contract signed in 2013.
Moved in when completed in 2015.
Moved out in 2019.
It has been rented ever since (2019-21).
I did not get a valuation done when I moved out.
Two questions regarding how much CGT will be calculated:
1. Can a valuation be completed now estimating its value in 2018 (when it became a rental)?
2. How does the ATO estimate its value? Is this simply dividing the difference in purchase and sale prices divided by number of years, then multiplied by how many years it was rented?

 

1. yes
2. you estimate its value. CGT is worked out using the cost base which will generally be reset to market value.

but it could also be CGT free potentially

 

Sorry I wrote my question wrong. What my point 2 was meant to ask is how does the ATO work out how much CGT to pay. ie is it done by dividing the difference in purchase and sale prices divided by number of years, then multiplied by how many years it was rented?