How the self assessment system works?

Under the self-assessment system, the claims a taxpayer makes in their tax return are accepted by the Tax Office, usually without adjustment, and an assessment notice is issued. Even though we may initially accept the tax return, the return may still be subject to further review.
To ensure the integrity of the tax system, the law provides the Tax Office with a period where it may review a return (and make sure all income has been included) and may increase or decrease the amount of tax payable. We may amend an assessment up to four years (or two years for shorter period of review taxpayers) after tax became due and payable under the assessment. Where anti-avoidance provisions apply, the period is extended to six years. Where the avoidance is due to fraud or evasion, there is no time limit on amending the assessment.