If this were where I am (ACT), it sounds very likely to be the unit owners responsibility to fix.
The Owners Corporation is responsible to repair and maintain plumbing that services multiple lots. The unit owner is responsible for lines that only service that one unit. So, somewhere, there will be a ‘tie point’ where the sewer line from the unit join the main line that takes input from multiple units. That is the point at which responsibility transfers (at least in the ACT). I always had the impression that a similar principle generally applied elsewhere. Even if the tie point is somewhere under common property and the individual tributary passes under both unit area and common property on its way to the tie point, the entire tributary is the unit owner’s responsibility.