Advance care planning (ACP) assists people to plan for their future health and personal care. ACP encourages a person to legally appoint a substitute decision maker (SDM) and to document any specific wishes regarding their future health care in an advance care directive (ACD). Formal documentation of wishes increases the chances that a person's wishes will be known and followed. However, one of the biggest impediments for doctors following the person's wishes is uncertainty surrounding the law, which is complicated and varies between the states and territories of Australia. SDM legislation varies regarding who can be appointed, how they are appointed, the powers that an SDM can be given and the decision-making principles that the SDM needs to follow. In circumstances where an SDM has not been appointed, the hierarchy for determining the default SDM for a person also varies between states. Although many states have legislated ACD forms allowing for documentation of a person's health care wishes, these forms allow for different things to be documented and have different requirements to be valid. The Australian population is mobile, with patients frequently moving between states. The status of ACP documentation created in a state other than the state in which a patient requires treatment also varies, with some states recognising interstate ACDs whereas others do not. This article outlines the legal status of ACDs, within Australian jurisdictions, including the legal validity of interstate ACDs, and argues that uniform laws and documents would assist with awareness and understanding of, and compliance with, ACDs.