Hi Willard,
I’d suggest also making a complaint to Sydney Water as they have the authority for sewerage matters. They won’t fix it in this case as I agree it is very likley the owners responsibility, but they might be able to issue some form of ‘notice to comply’ on the owners.
For your information, the owners have responsibility for their pipe all the way to the connection to the main in the street. This responsibility is often misunderstood by owners and there is a misconception that it becomes Sydney Water responsibility when it leave the owners property.
Good Luck,
Rob T