You can download the CO form from the internet, it is usually preferable to do that, fill it out, and take it into your GP or whoever is going to sign it, I have heard of people being given grief for even asking for the form from the GP's office and then being harassed by staff at the time, or by phonecall ...

www.medicareaustralia.gov.au/public/files/ma_conscientious_objection_form.pdf