The appointment of a guardian only has effect at such times as the person is "in need of a guardian".
Under the Guardianship Act 1987 (NSW), a person may chose in advance who they would like to be their guardian if one is needed. Such an appointed guardian is known as an "enduring guardian". This must be done in writing, and the document must be to the effect of the form contained in schedule 1 to the Guardianship Regulations 2005 (NSW).
Depending on the content of the appointing document, an enduring guardian will be able to decide where the person who appointed them lives, decide what healthcare they receive, decide what personal services they receive, give consent to medical or dental treatment, and perform any other functions relating to "their person" specified in the document.
Conditions or limitations can be placed on the enduring guardian(s) power by specifying those conditions or limitations in the document. This includes excluding authority to exercise a function, as well as giving directions as to how a function is to be exercised. You can extend (as well as restrict) the authority of an enduring guardian by specifically stating so.
An enduring guardian is authorised by the act to, on their appointor's behalf "sign and do all such things as are necessary to give effect to any function of the enduring guardian".
For more information on appointing an enduring guardian see the Guardianship Act 1987 (NSW) or download the documents Enduring Guardian Information Sheet and Guardianship Act Summary