For the purposes of the definition of Maunganui Bluff–Tirua Point set net prohibition area, a straight line joining the mean high-water mark at each side of a harbour entrance or, as the case may be, the natural entrance points of a river mouth must be treated as the inner boundary of that area if it extends seaward or as the outer boundary of that area if it extends landward, except in the case of any harbours or rivers for which specific co-ordinates are provided.