In 2019, the Tasmanian Parliament made major changes to the law, allowing a person to register a gender (which could be male, female, non-binary or another descriptor) and thereby to amend their birth certificate. This could be based upon nothing more than self-identification. Such laws have been enacted elsewhere in Australia and in other parts of the world. It is argued that the new laws reflect ideas about gender that are in radical discontinuity with the medical model on which the law concerning registration of a change of gender was previously based. The Tasmanian legislation is of international interest because it goes further than other laws by stipulating that a person who changes their gender identification is, for the purposes of Tasmanian law, to be treated as being of that gender. It follows that if someone declares themselves to be 'non-binary' or 'agender' then for a great variety of legal purposes, they must be so regarded. The difficulty is that these new ideas have been grafted onto a statute book which assumes there are only two sexes, male and female. The result is legislative incoherence. There are lessons in this for other jurisdictions around the world. Policy decisions have to be made about when distinctions in the law should be based upon biological sex and when they should be based on self-identified gender. The whole statute book needs to be considered in order to ensure that changes create consistent effects across different areas of application.