We are the higher propery, with the property below that are new owners (1 year). We had built a retaining wall of 1.5m above the natural slope of the land in the 1970s when we built the house (have photos). This year, we've notice subsidence in back yard near retaining wall. I cut a path through the tree covering the area and vines that my previous neighbour planted at the back with owners permission to investigate. It is apparent that the wall has been undercut by an average of approx a whopping 3 metres to achieve a flat ground level for their shed that occupies the space. So now the earth to retain at boundary is 4.5 m. The waterboard pipe is just inside our existing retaining wall. With an inspection pit that is suspended in space just outside the retaining wall by a rock outcrop. All this used to sit on the natural slope of the land. The cost of building a new retaining wall to this new height has been quoted as around $250,000 (including waterboard requirements). Apart from the fact that you need waterboard approval to excavate near a pipe (i.e. illegal actions). This negligent unsupported undercutting has undermined the foundation of our existing retaining wall and caused subsidence, and increased the wall replacement height from 1.5 to 4.5m. How do I proceed? Any help would be much appreciated.