It would be a good idea to consult the strata plan in order to verify your understanding that you have an entitlement to access the terrace from your balcony.
Why does the strata committee wish to upgrade the balustrades to the current building code? Usually this is not necessary, unless the balustrades are obviously unsafe (for example missing or too wide spacing between balusters) or other works have necessitated a DA and the Council has conditioned an upgrade. Also, works subject to the new Design and Building Practitioners Act can have the consequence of upgrades being made.
If as you say your balcony is less than 1 metre above the ground, there would be no necessity for any change to your balustrade but for matching the rest of the building?
The strata committee would require approval by way of a special resolution, that is not more than 25% of votes against the resolution, in a general meeting, in order to proceed with the project.