New Zealand Law
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The law about retaining walls is that you have the right not have your property eroded by their work on their section, and you have a right of support. So if they excavate and this causes the failure of the wall then they should put this right and pay for the wall. Fences are a shared cost, but the retaining wall is an issue where they have caused the problem, and therefore need to ensure your property does not slip on to theirs.