The Harbourmaster has announced a proposed 10 knot speed limit for all vessels, except for ferries, emergency and police vessels and racing yachts, in Auckland's Waitemata Harbour.
There is a regular harbour 5 knots speed rule for when within 200m of the shore or within 50m of another boat already in existence which is not policed and creating another law is going to do nothing to prevent accidents happening on the harbour. Uneducated or uncaring boaties are the problem. The owners of large displacement launches must be made aware of the dangers that their wake causes to small craft, speeding vessels their danger to hard-to-see kayakers and all users must be aware of and bound by the "rules of the road" (the colregs) at sea.
Stop wasting your time making new regulations - just police those in existence.