The Land Transport Act 1998 - Section 64.

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.

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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.