This is a tricky one becasue it’s a building defect that should have been fixed in the first 6 years of the building’s life, but is it common property? I don’t think tiling and waterproofing on an internal wall is common property (happy to be corrected on that if I’m wrong).
So my take would be to go back to the committee and offer to share the cost of repair and remediation on the grounds that if they don’t, you will run this through NCAT just to get a definitive ruling. Half the cost of repairs against all of the legal costs and maybe all of the reapirs might be a gamble the committee doesn’t want to take.