This paper analyses the efficiency of the Australian tax system using CGETAX, a large-scale, long-run CGE model designed for tax policy analysis. This follows an analysis with CGETAX of an Australian Government proposal to reduce the corporate tax rate from 30 to 25 per cent in Murphy (2016a) and Murphy(2016b) and an Australian Treasury Working Paper on the efficiency of certain taxes (Cao et al., 2015). This paper begins by uses a highly stylised version of CGETAX to provide a theoretical analysis of the efficiency of major taxes, applicable to advanced, open economies in general. The Stylised model, like CGETAX, allows for imperfect competition and models the disincentive effects of taxes on labour supply, the capital-to-labour ratio, and the choice between present and future consumption. Of the major taxes, company income tax is found to be least efficient, with a marginal excess burden of 139 cents per additional dollar of tax revenue. For open economies the literature finds that company tax is among the most inefficient of taxes because it suppresses labour supply and the capital to labour ratio and leads to profit shifting to lower taxed jurisdictions. For Australia, company tax is even more inefficient because of its above-normal company tax rate and the erosion of the final revenue yield through the system of franking credits.