HOA's are not like Sectional Title Units, where there is an act that says what the members of the body corporate can do and cannot do. The Act also explains the remedies of the body corporate/trustees.
HOA's are responsible to draw up their own documents in that regard. This is normally the HOA's constitution and other policy documents or rules. There is, for instance, no uniformity rule in an HOA unless such a rule is part of any of the above documentation. Same with the Management Rules in the Sectional Titles Act that require the owners to maintain their units.
So, in order for me to answer that question, I would need to have a look at the documents applicable to the HOA and make sure that it is valid. I will assume for the moment that it is indeed valid. You would then have to go through the valid documents and see whether the obligations that you want to impose on the owners are actually imposed by the constitution or rules. If not, there is very little that you can do. If it is in there, you can proceed to step 2.
You need to check the very same documents to establish the procedure to address a breach. There may be notices that you are required to send in a particular way. There may be clauses that says you must refer the matter to arbitration. If there is no such clauses, then you would have to approach a High Court (advisably with the assistance of an attorney) to obtain an order to compel them to fix their units and demolish unapproved structures.
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