Masked2014, Jimmy & Whale,
has anyone thought about the simplest and most obvious of all by-laws, normally by-law 5 under schedule 1 by-laws, being that you can not damage common property. I would envisage that it would be easy to prove that no other apartments have down lights installed, penetrating the fire separation or cutting ceiling joists.
further to this, you may find that there is some considerable legal precedent around this area, whereby an owners corporation has taken action against an owner for not having an exclusive use by-law in accordance with s52 or 65A of the act for alterations to common property, being the only mechanisms in the current Act to permit an owner to alter common property and to keep these alterations to common property. It may be possible to come to an agreement with the owner/OC to permit the alterations in retrospect.
Masked2014, sounds like you may need to seek professional advice from a qualified building consultant and or make application to Fair Trading in relation to the defective building works.
mr s