little voice – I don’t know in what forum the Owners Corporation (O/C) has said “no” to your request, but that decision may only be made at a General Meeting and not by your Executive Committee, which I suspect may have been the forum in question (?).
By all means check your Title Plan, but I agree with the others here who’ve suggested that the existing garage doors would almost certainly be common property, and therefore any request by you to replace those would need to be decided by the O/C as a whole, where ≥75% of those Owners in attendance at the General Meeting would need to vote in favour of your proposal in order for consent to be granted.
So submit a formal Motion to your Secretary (or to your Strata Manager if that role has been delegated to them) outlining what it is that you propose and why, advise that you are prepared to meet the cost supplying and installing of the roller-doors, and that you’ll maintain them thereafter, and perhaps lobby other Owners between now and that Meeting so that you can achieve that 75% majority vote.
If your proposal is consented, then strictly speaking your O/C should draft and register a Special By-Law (SBL) to formalise your maintenance, repair, and replacement responsibilities for the roller-doors, and if it’s likely that other Owners will in future make future requests of that same type (as yours), then the O/C should cover the costs of that SBL.
If you don’t receive consent, then polish your walking stick and apply for mediation of your proposal, and depending upon how that goes you can at your discretion apply to for the matter to be adjudicated upon by the NSW Civil and Administrative Tribunal.
A lot of rigmarole for what should be a simple procedure I agree, but that’s how it is in NSW I’m afraid, where there’s no differentiation between minor changes to the Common Property such as yours, and major ones such as when Owners propose to significantly renovate their entire Lots.