Australia and New Zealand have an ambitious intention of driving towards a trans-Tasman Single Economic Market (SEM) based on common regulatory frameworks. Tax harmonisation is a key part in the process of becoming a SEM. The Australian and New Zealand tax systems share a common culture in many ways. This dissertation examines the possibilities and difficulties for Australia and New Zealand in putting aside tax competition to harmonise their current tax systems under a SEM. The researcher carried out a secondary analysis and quantitative data analysis to gain insight into the influences on tax harmonisation. The data resources were mostly retrieved from the official statistics websites of both countries. This research took an economics approach in analysing tax incentive and tax administration. It gives insight into the income tax model and current tax system in New Zealand and Australia. Based on the principles of a good tax system, and the differences in both tax systems, the researcher provides a range of suggestions for tax harmonisation of the tax base, tax entities, tax rates and tax rules. In terms of tax policy innovation, the study concerns the effects of reforming existing tax policies instead of adopting streamlined agreement. For reasons of length, this dissertation focuses only on the structure of the tax systems rather than the wording of the tax legislation.