Soil fall on gradient land

As you can see in the photos, the builder has added a retainer wall up to a certain point, after-which the soil has nothing holding it and is already starting to drop.

Apparently starting from where the retainer wall ends, that all council land.

My question is, was the builder supposed to make sure no land escapes into another lot, etc, making this a defect. Or has the builder done what they are supposed to and everything here is compliant, at least regarding the bit Im concerned about?

Thank you