Here is what the official blurb from the Government website says:
The two witnesses must:
• be at least 18 years old
• not know or believe that they are a beneficiary under a will or otherwise going to receive a material benefit from the patient’s death
• not be an owner or day-to-day manager of the health facility in which the patient is being treated or where they live
• not be directly involved in providing health services or professional care services to the patient.
Witnessing a patient’s written declaration does not require prior knowledge of the patient or specialist knowledge but is based on the witness’s observation of the patient at the time. Someone in an administrative role or other role who is not directly involved in the patient’s care could be a witness.