There are few misconceptions in this post – no criticism, you are not alone.
Firstly the “warning letter” is called a Notice To Comply and you will find the official (and only acceptable) form HERE.
The form, quite reasonably, asks you to state the by-law that is being breached. If you don’t have a by-law (such as one about behaviour on common property) then you have a problem.
If you issue an NTC and it is ignored, your next step is to go to the tribunal (NCAT) to seek penalties and/or orders. With a properly issued NTC – agreed at a committee meeting or issued by a strata manager – you do not need to go through Fair Trading.
If you don’t have an appropriate by-law, then you probably do have to go through the Fair Trading mediation process before you can go to the tribunal.