As per Australian guidelines, the standard requirements for a medical certificate are: 
 

• Name and address of the doctor issuing the certificate
• Name of the patient
• Date on which the examination took place
• Date on which the certificate was issued
• Date(s) on which the patient is or was unfit for attendance
• A diagnosis is not usually required. If it is a requirement for a diagnosis to be provided, the doctor should inform the patient and only proceed to include the diagnosis following the express consent of the patient.

People are sometimes incorrectly advised by employers or others that getting a medical certificate relating to earlier dates is not allowed. However, Australian employment law states that it is allowable for employees to obtain a medical certificate shortly after the dates in question. According to www.fairwork.gov.au : “an employee has to let their employer know that they are going to take sick or carer’s leave. This has to be done as soon as possible, and can be after the leave has started. They should also specify how long they will be off or expect to be off work”.

Subject to assessment and clinical judgment, a doctor can certify that a period of illness or injury occurred prior to the date of examination. This means the date of assessment appears at the top of the medical certificate, and the dates of the sick leave are stated within the text of the certificate- thus the medical certificate can state that you were unwell on the earlier (or upcoming) dates in question, though the date of your consultation must also appear on your medical certificate (that’s a legal requirement).