I believe that if the OC has given permission for someone to build on common property, without an exclusive use by law drawn up, then the OC is responsible for the maintenance, upkeep etc.
Did this owner get permission or not? He would have to have council permission to build and council would want OC permission? Even if you lived in a house you would need To check council guidelines.
If there isn’t an exclusive use by law for this owners construction, I would suggest your complex get one.