If you choose the rollover, all or part of the capital gain will not be included in your assessable income until a change in circumstances happens - for example, you don't meet the rollover conditions within the specified time period. One of the rollover conditions is that you must acquire a replacement asset or incur capital expenditure on improving an existing asset.
If your use of the replacement or improved asset changes this is a change in circumstances - for example, you no longer use it in the business, or you sell it.
When a change in circumstances happens, the deferred capital gain will crystallise - that is, all or part of the capital gain previously deferred will become assessable.
You can choose to apply certain concessions to this gain. If there is any amount of the capital gain left after applying all the concessions you choose, this amount will be included in your assessable income.
Cost base of replacement asset
There is no reduction in the cost base of the replacement asset when the capital gain is rolled over.
Example
Karen had a capital gain of $100,000. It has been reduced to $25,000 under the CGT discount and 50% active asset reduction. She chose to roll over part of the remaining gain into an asset she has owned for eight months. The total of the first and second elements of the cost base of the replacement asset is $20,000. Karen can disregard $20,000 under the rollover, leaving a final capital gain of $5,000.