Who pays for sewage leaks in your street

[fyfin]

We live in an entidad which is not the same as a community, basically, it means roads etc are still public but because the estate has not been adopted by the local town, then the entidad is responsible for the upkeep, lighting, road markings etc. We pay as owners towards this but not a huge amount.
What is confusing me is that we have been told that if there is a blockage (in this case due to bad construction of a drain junction in the street) and raw sewage is flowing into the street, then the owners are liable.
These junctions are are shared by the two adjacent houses, so each set of two houses has a manhole that is shared.
What I can't understand is how you can be liable for something beyond your boundary and I suspect you can't insure against it although I have asked the question.
The problem is not with us but as we live just meters away we have to negotiate around or through it and it's not very pleasant. If it was outside my house I would pay anyway just to avoid the unpleasantness but I wonder if anyone else has an opinion or have come across this before.
I think we had a similar issue in town when the Ayuntamiento and Galasa had to share the cost of fixing a drain that was putting raw sewage in the street.