We have a property lower than our neighbour's retaining wall. It has been confirmed the wall is theirs through surveyor, council plans and land contour maps. The original owner filled the property to be level. During that 12 months we have excavated raised garden beds on our property and the operator left approx. 900mm from their retaining wall to not disturb the wall while we proceeded with renovation. We have had concrete sleepers and steel uprights delivered to be installed where our garden beds were and not touch neighbours wall. Ours has been engineered. During recent Cyclone Marcia rain the neighbours wall and Colorbond fences collapsed down onto out property. We are all working amicably to get the wall sorted and agreed we need to complete ours in order for them to do theirs higher as our backfill will support their new uprights better. The heated conversation now is his argument is we excavated into natural land near his wall so the failing wall is our fault. Our argument is your wall has failed over last five years, collapsed in areas we have excavated near and now that we inspect the damage we see his 1.2m (above ground) uprights were only in the ground by 60cm and back fill was ash mix which absorbs water and can cause more pressure behind his wall. Is there a law that states we may be responsible for his poorly built wall due to excavation work being done near his wall? He has told me he will get the wall fixed but I'm worried he will try and sue us for costs afterwards.