The Land Transport Act 1998 - Section 64.

If the infringement you are disputing is a result of a blood test, it says:

Section 64(1A) - It is a defence to proceedings for an offence against section 57A(1)* or (2), 57B(1)* or (2), 57C(1)* or (2), or 62(1B)* if the Court** is satisfied that the person has consumed the relevant [qualifying drug]—

1. in accordance with—
   1. a current and valid prescription written for that person by a health practitioner; and
   2. any instructions from a health practitioner or from the manufacturer of the [qualifying drug]; or
       
2. because it was administered by a health practitioner, provided that the person complied with the instructions (if any) that the health practitioner has given.

If the infringement you are disputing is a result of an oral fluid (saliva) test, it says:

Section 64(1AB) - It is a defence to proceedings for an offence against section 57A(3), 57B(3), or 57C(3) or (4) if

1. the person—
   1. has a current and valid prescription for the qualifying drug that was written for that person by a health practitioner; and
   2. has complied with the instructions (if any) from a health practitioner or from the manufacturer of the qualifying drug about driving, consuming alcohol or other prescription medicines, or both, while consuming the qualifying drug; or
2. (b) the drug was administered by a health practitioner, and the person complied with the instructions (if any) given by the health practitioner.

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*Offences under these sections (57A(1), 57B(1), 57C(1) & 62(1B)) are criminal charges and do not relate to infringement offences. If the offence you are alleged to have committed falls under one of these sections, you should seek professional legal advice.

**Offences under 57A(2), 57B(2) & 57C(2) are infringement offences and therefore a medical defence can be considered by the Police Infringement Bureau.