According to the Victorian health department website 12-17 year olds can provide their own consent to the clinical trial covid 19 injection without parents involvement (as opposed to 5-11 year olds who must have parental consent).

Without going into the inconsistencies of the law of age of consent and how just plain ridiculous such a proposition is in my personal opinion, here is what you can do on behalf of your child who does not consent:

If a child aged 12, 13, 14, 15, 16 or 17 yrs can consent on their own to taking the covid19 according to the government guidelines, then presumably they will be signing their consent on a form at the time of taking the jab. Therefore, arguably, they equally can sign a non consent form which can be served by proof of receipt email to doctors, school teachers, school principals, other parent, etc etc

Here is a such a form that can be signed by the child (and parent(s)