Marvin – firstly, it’s always best to manage strata “by the book”, particularly so in your case as in the absence of a prior consent to your aircon system by your Owners Corporation (O/C), it could successfully obtain Orders from the NSW Civil and Administrative Tribunal requiring you to remove the system and to reinstate the common property.
The correct procedure is for you to formally seek the consent of your O/C to the proposed aircon installation by placing a Motion to that effect on the Agenda for your next General Meeting, where ≥75% of those Owners in attendance would need to vote in favour for your Motion to pass.
Then once your Motion passes, and if your O/C wants to make you and subsequent Owners of your Lot wholly responsible for the maintenance and repair of the aircon system and that part of the common property to which it’s attached (i.e. the wall), then it should concurrently resolve to create and Register a Special By-Law in those terms, inclusive of the general conditions covering the installation of your aircon system.
As for the costs of drafting and Registering the Special By-Law, the latter cost $115 so in your case there’s a quoted fee of $885 to draft the words, which is probably reasonable if there’s a Lawyer involved.
However, unless there will never be a circumstance where other Owners will seek the O/C’s consent to install aircon systems (unlikely?), then it’s entirely reasonable for your O/C, which includes you by the way, to pay for the drafting and Registration of a generic Special By-Law covering such installations.
This would give your Executive Committee the ability to itself grant future consents, and thereby avoid a Motion at a General Meeting on every occasion that an Owner wants to install an aircon system and further Special By-Laws of the same type (and presumably further $1K payments by Owners) for each consent.
On the other hand, if you want to pay for that generic Special By-Law ……