Medicare does not offer any rebates unless a personal consultation between doctor and patient has occurred - therefore, absentee requests for certificates, referrals or prescriptions must be paid for privately without any rebate. Although your payment details are collected at the time of lodgment, payment is only taken if Dr Dan can fulfill the request.
Regulators forbid the backdating of medical certificates (for example, a certificate written on 2nd May to be dated as 1st May would be unlawful). However, a certificate can still *reference* past events / dates, so long as its own date is current (for example, a certificate written on 2nd May, dated 2nd May, stating "patient was absent from work on 1st May", is allowed).