Political commentators have long said that the enactment of a comprehensive capital gains tax (CGT) in New Zealand (NZ) would be political suicide. However, sentiment towards a CGT in NZ has softened more recently with a number of commentators as well as business and political leaders supporting the introduction of a CGT. In both the 2011 and 2014 General Elections the centre-left Labour Party campaigned on inter alia introducing a comprehensive CGT levied at a rate of 15 per cent. They lost in both elections but on the basis that a CGT is now clearly part of the NZ political agenda of the left-of-centre parties coupled with the current mixed member proportional (MMP) electoral system (which lends itself to coalition-based governments) it is arguably only a matter of time before a CGT is introduced in NZ by a Labour-led coalition. This paper, using Adam Smith’s canons for a good income tax, considers the Labour Party CGT proposal and the United States (US) experience in dealing with capital gains. It notes that the NZ proposals are consistent with CGT regimes generally, such as it being realisation-based and not permitting indexing, but may involve complexities and pitfalls that the US system has previously encountered.