Waterproofing the external concrete slab to rectify leak

Fellow strata community,

Can anyone confirm if the waterproof membrane is common property on a cantilevered balcony concrete slab?
In this situation, we are a strata scheme registered prior to 1 July 1974.

Currently, under negotiations with the SM and EC to rectify a leak, with replacement of the original timber frame windows/door.

The builders have said a lack of waterproofing to the balcony and lack of step down is a contributing factor to the water ingress.
Today the SM has said at the AGM that the trafficable membrane is not common property. He stated that it was not present at the time of registration of the plan. It will not be covered by OC when breaking down who is responsible, so I will wear almost the full cost, with only the magnesite removal and its replacement being their responsibility.
Currently, there are traces of a coating which is uniform across the balcony slabs. The slab itself is common property. Why not the appropriate measures, according to the building code to protect it?

Hoping someone can assist in some way.
Many thanks