This study explores the impact of complexity upon unintentional non-compliance behaviour for personal taxpayers in Australia. This area of research did not appear to have been previously studied in an Australian context and in this respect, the study represents an original contribution. While studies have been conducted both in Australia and overseas, they have generally been directed at other types of compliance behaviour and tend to be inconclusive in their findings. According to the compliance literature, there appeared to be little consensus of opinion on the factors that determined behaviour and appropriate research methods. It emerged that more narrowly-defined studies with stronger research methods offered potential for furthering knowledge in this field. Thus the study focused on one behavioural outcome and one type of taxpayer, using a multi-paradigm research method. Unintentional non-compliance, as an outcome, was selected as it appeared to hold promise for improvements in overall compliance to be readily made, provided its causes were understood. Complexity was considered to be the most likely cause of unintentional non-compliance, and those who prepared their own income tax return, the group likely to be most affected. The study used both a quantitative and qualitative component from which a number of convergent results emerged. These included that the major cause of complexity was the ambiguity of tax laws and the volume of explanatory material required. Further, personal taxpayers were committed to compliance even though they regarded the system as less than fair. Together, complexity and commitment to compliance caused taxpayers to experience unnecessary compliance costs. Where taxpayers completed their own return, complexity resulted in a high level of errors that generally resulted in an overstatement of tax liability. In addition, some taxpayers chose to be over-compliant as a means of dealing with complexity and commitment. It was concluded that complexity compromised the integrity of the Australian income tax system by imposing an unfair burden on personal taxpayers in respect of both tax paid and compliance costs incurred. However, there appeared to be little, if any, financial incentive for the tax authority to address the causes of complexity for personal taxpayers.