Bernie – In Victoria the Owner of the Lot with the balcony will generally be responsible for the repair of the membrane. See here: https://www.propertyprofessionals.net.au/wp-content/uploads/LMS-Legal-Tiles-and-Membranes-A.pdf
Being that the membrane repair is not a body corporate repair then I would suggest that it is the Lot owner of the affected Lot who would take the matter to VCAT. See here for a recent case where the affected Lot owner took the matter to VCAT against the unresponsive Lot owner: https://www.austlii.edu.au/cgi-bin/sinodisp/au/cases/vic/VCAT/2016/2059.html?stem=0&synonyms=0&query=balcony%20replacement%20and%20owners%20corporation
The OC can always provide an input into the argument by outling the procedure that they have undertaken to resolve the dispute.