Johnno – you’ve hit the proverbial nail on the head, in that what’s been done consequent to rectification works is an alteration or addition to the Common Property, that in NSW should have been pre-approved by a minimum 75% majority vote as determined from the unit entitlements of those present at a General Meeting of the Owners Corporation; so that’s what you’re looking for.
The Owners of the three (3) top-floor Lots now have exclusive use of part of the Common Property, and contrary to the advice that you’ve received, that has likely added to the value of those Lots and may have diminished the value of yours due to the privacy issue that you mention.
Again, the exclusive use matter should have been incorporated with the Motion to consider the alteration / addition to the Common Property, together with an enabling Special By-Law to amongst other things require the three (3) benefiting Owners to make regular payments to the Owners Corporation that reflect the “value” of that use, which would in normal circumstances be reflected by their Levy Contributions.
Before discussing what can or should be done about all this, may I suggest that you come back after you’ve found out how it is that the works were approved and funded, because advice in that regard will depend upon how that process was handled.