Is it in your contract that you need to state the illness/treatment on the certificate?

Maybe you could ask your FS what they think about it and are you within your rights to not have it written on the certificate?

Here is some information from the Smart Occupation Health & Safety website outlaying questions and answers about sick leave, it may help out a bit more.
http://www.smartohs.com.au/site/files/ul/data_text30/899973.pdf"]http://www.smartohs.com.au/site/files/ul/data_text30/899973.pdf