It's a COLREGS treaty clause. I can't find any NZ case law.  But there's several case law examples from overseas jurisdictions that have held that an anchored boat has been atleast partly liable during collisions, when both boats have been anchored and also where only one boat was anchored. At least one of these cases went back to 1921... and it's a 1970s treaty...  NZ courts would look to these cases for definition. It's unlikely it would ever even make it into a NZ court because insu