So what are the rules around taking sick leave?
Under the Fair Work Act, sick leave is now bundled up into what is called "Personal leave". Unless you have different provisions in your industrial instrument (i.e. your contract of employment or Enterprise Agreement), employers can ask for a medical certificate for any absence - even just one day. In practice, most employers have a 2 days or more rule unless there is a pattern of absences or excessive absenteeism.
If you are sick you should call your Manager or nominated person at your work to let them know including when you expect you will be able to get back to work. If you start taking every Friday (or Monday) off, or start taking A LOT of sick leave it is likely you will be asked to attend a meeting. Things can start getting tricky at that point, so the best thing you can do it only call in sick when you really are, and follow the requirements around medical certificates. If you find yourself in a position where you know you will need to take a lot of sick leave then just be honest with your employer and let them know what is happening. In my experience 99.9% of Managers are just concerned about your health and well being.