Generally no, unless it is from your regular doctor who you have seen in person in the past 12 months. A legitimate certificate requires a consultation with a registered practitioner, and Medicare rarely covers telehealth with a provider you have never seen in person. Free routes that exist offline include a bulk-billing regular GP, student health services, community health centres, or, for some workplaces, a statutory declaration instead of a certificate.
However, there are several affordable same day medical certificate options in Australia, such as Qoctor.