The first thing that you should do is to obtain a copy of your Plan’s Special By-Law (SBL) that grants exclusive use privileges to the penthouse Owner, and look for any provision that requires that Owner to maintain any or all of the roof-top.
If the SBL is silent on the subject of maintenance responsibility, then your Owners Corporation (O/C) retains responsibility for that activity, including on the balustrades, and I’m afraid that unless some procedural requirement of NSW Strata Schemes Management Act (the Act) was not properly followed when the exclusive use privilege was originally granted, there’s nothing that your O/C can do about that privilege, of any other provision of the SBL, or to even seek to amend or repeal that SBL, without the prior written consent of the penthouse Owner.
On the basis of the information in your post that consent won’t be forthcoming, so the only avenue that your O/C then has to at least have the penthouse Owner make Levy Contributions that better reflect the increased “value” (incl. market value) of their Lot consequent to their exclusive use of the roof-top is to have a Registered Valuer determine a Revised Schedule of Unit Entitlements for your Plan (and for the penthouse Lot specifically).
The means to do that is relatively easy if the Plan has been Registered for <2 years, but as yours is an “older block”, I’m afraid (again) that the only feasible option is for your O/C to seek Orders under Sect 183 of the Act, which again should be supported by a Valuation of the penthouse Lot.
The situation that you describe is but one reason why the granting of exclusive use privileges needs to be very carefully considered before a vote is taken.