With all assets you need to keep records. In this case, you will need to keep records of the transactions or events that provide evidence of your assessment of how the main residence capital gains tax exemption applies to the part of your main residence that has been compulsorily acquired. This includes a record of your calculations of your capital gain or loss and if your property is greater than 2 hectares.
To find out more about the recordkeeping requirements in relation to assets and capital gain tax see Record keeping for CGT.