The period in which the Commissioner of Taxation can amend assessments is generally either two years (for small businesses) and four years (for non-small businesses). It is important to note that the clock starts ticking on the day that you receive your notice of assessment.
Example: Where you receive your notice of assessment from the ATO on the 1 August 2017, the two-year amendment period begins on 1 August 2017. This means that the two-year period will lapse on 1 August 2019, two years from the date in which you received your notice of assessment.
However, there are important exceptions to this general rule. For example, where the Commissioner is of the opinion that fraud or evasion has occurred, an unlimited period of review applies. Certain provisions, such as section 100A of the _Income Tax Assessment Act _1936 also contain unlimited review periods.