Kenny – as I suspected, the intro. wording that you’ve now provided confirms that where decks are in place, everything from 1 metre below their surface to the underside of the first floor eaves of the respective Lots is common property, and the responsibility of the Owners Corporation (O/C).
There are only three (3) ways for your O/C to absolve itself of that responsibility.
The first is by way of Sect 62(3) of the NSW Strata Schemes Management Act (1996), the second is by way of Sect 65B where each Owner is granted exclusive use of the decks (under the rationale that nobody else could use them anyway), and with the Condition that each of those Owners is responsible for the maintenance and repair of those, and the third is by way of Sect 65(A) 1(b) where the O/C could simply remove its deck/s.
All of the above requires a Special Resolution to pass at a General Meeting, where ≥75% of those voting (personally and by proxy) would need to do so in favour, as determined from the units of entitlement (UOE) of each, and the total UOE of all those voting.
Regrettably, the first of the above is additionally conditional on the O/C’s decision not affecting “the safety of any building, structure or common property in the strata scheme or detract from the appearance of any property in the strata scheme”, the second requires written consent by each of the affected Owners followed by the Registration of an enabling Special By-Law, and the third would probably need to involve all decks and not just the one that’s at issue.
I doubt that any of the above would easily occur in the circumstances that you describe, so the best solution in my opinion would be for your O/C to allow an additional amount in its next budget to specifically cover the maintenance and repair of decks, so that at least every Owner is then aware of and transparently contributing to that, and for the O/C to then bide its time until an Owner seeks its consent to alter or change a deck, such as by roofing it or placing a new/different railing around it, at which time a Condition of any consent by the O/C could be that such Owner/s maintain and repair the entire structure.
Sorry – not the answer that you wanted I’m sure!