I’ll assume the work can be done safely from within the balcony. Otherwise it should all be done professionally.
I expect there would be some mechanism in the Act in whichever state you are in to enable unit owners to be billed for this separately and other than in accordance with unit entitlements but it might require a general meeting resolution. It would in the ACT.
Assuming it can be done, why not present two options along the following lines:
‘1) If you have the time and inclination, prefer to save some money, and prefer not to have to arrange a time for painters to enter your unit to access the balcony, the committee will supply a pot of paint and instructions about what to paint. However, if the work is not done by (some date), the committee will arrange to have it done and bill you as per option 2.
2) If you prefer to have it all done for you, the OC will bill you pro rata as one out of however many take this option. Assuming that at least X units take this option, the cost per unit will be $Y and you will need to allow access to your unit on a date yet to be arranged.
All owners will end up paying for the paint via their levies. The difference is only whether you pay for the labor or avoid that by doing it yourself.’
Note that if a survey finds that there are only very few units in one camp or the other, I would not bother with the above. If nearly all are in the DIY camp, then I suggest that a few people just offer to paint the railings of their elderly neighbours. If nearly all are in the ‘just pay for it and get it done’ camp, then the few DIYers will have to just accept that it will all be done by a professional painter and it will be covered in their levies.