Assuming your Plan’s in NSW and no contributory negligence on the part of the Owners Corporation, then it’s responsible for rectifying the sewerage blockage downstream of your unit’s overflow gully together with any damage that’s consequently caused to other areas whilst it’s doing that (including to your unit), so save any ex-gratia payment that the O/C may make to you, you’re responsible for rectifying the damage to your unit via your contents insurance or otherwise.