Felix – as both Scottie and myself advised, the Owners who have installed the air-conditioner are not in breach of any By-Law and therefore a Notice to Comply cannot be issued!
Again, the learned Scottie and me advised you that those Owners are in breach of Sect 65A of the Act, and in that regard you’re quite right by suggesting that you and like-minded Owners need to quickly sort this out, because in my experience the longer such breaches of the Act are allowed to continue, the more quickly others perpetuate, and the more likely they all are to become “rusted-on” (pardon the pun).
In the absence of a relevant Special By-Law at your Plan, the Executive Committee (E/C) cannot itself grant consent to the air-conditioner installation (which whether by design or by coincidence they didn’t), and as only a special resolution at a General Meeting can grant that consent (or not), the matter needs to listed as an Agenda Item at that forum.
If your next AGM is too far away, then you need to speak with like-minded Owners and if necessary lobby more to the extent that you have at least 25% of those (by unit entitlement) on-side, at which point you can collectively “requisition” the E/C Secretary to convene an Extraordinary General Meeting just to consider and vote upon granting conditional consent to the air-conditioner installation, or otherwise to instruct the offending Owners to remove it and to return the common property to its original state.
If your Owners Corporation (O/C) does grant consent retrospectively with conditions, then as it would presumably want to enure that the Owners and not the O/C are responsible for the maintenance of the entirety of the installation, it would additionally require a Special By-Law to be concurrently voted upon in those same terms (i.e. a special resolution) in order to achieve that.
This is getting a little complex, so maybe come back if / when the General Meeting is about to be convened.