From a slightly different perspective, be aware that once you start an action against the OC you will need to stay the course and hold your ground. They may have ways of wearing you down and pressuring you into giving up.
If the OC dig in aginst you, which from your post appears likely, you may find that your roof repairs get dragged out for a long time.
NCAT hears many cases, there is a long lead time to get a hearing and orders and going without a lawyer will not help. Lawyers fees may exceed the roof repairs. (See the whoopi case)
Also be awarec that NCAT don’t award costs under 30k unless there are specific grounds to do so.
What I would do before gping down that road is to try to negotiate to get the repairs done.
I would tell them you will consider a contribution towards the repairs, but you are only willing to pay it after the work is completed.
When the work is competed tell them you had advice that you can’t contribute towards common property repairs as a seperate contrubition as there is no by law or special levy in place that makes you responsible for payment or ongoing maitenance and refuse to pay.
Ps. Don’t sign anything. This is not legal advice and there are no refunds.