Retaining wall in easement

Hi All
We recently built a house in Sydney through a land and house package. Before signing the documents, the builders sales representative told us that we’d have to build a retaining wall on the one side of the house after handover. He mentioned that it’ll be a small one but they don’t do it. The lot was deeply excavated and now we have to build a retaining wall pretty much the entire length of the house. The problem is that there is a service & maintenance easement that runs half way down our house on the same side.We’ve been told that we can’t build a retaining wall in the easement. The neighbour hasn’t started to build yet but has advised that he’ll be using a drop edge beam on his side so won’t need a retaining wall. No idea what this means! Can anyone please offer advice? Been to the council/certifier/developer without any luck.