The Auckland Harbourmaster is directing skippers of power-driven vessels to observe a 12-knot speed limit in the inner Waitemata Harbour.
Auckland Regional Council Harbourmaster John Lee-Richards said the limit would help some skippers who had difficulty assessing a maximum safe speed for increasingly busy waters.
He said therehad been a high number of collisions.
In all of them, the skippers of power boats travelling at speed had not seen the second vessel and the risk of collision.
They hit slow-moving smaller craft which could not get out the way in time.
"The Waitemata gets busier each year and everyone needs to slow down and keep a good lookout."
Mr Lee-Richards said he was also concerned about the use of automatic steering devices in high-traffic areas where fast reactions were needed to avoid collisions.
The use of these is banned within the inner harbour speed limit zone, which stretches from the harbour bridge to North Head.
The ARC earlier hinted at a 10-knot restriction but boaties said 12 knots would let boats get their hulls to plane.
Royal New Zealand Yacht Squadron commodore John Crawford said members would be encouraged to observe the directions, which applied from the weekend.
No restrictions apply to the south side of the harbour inward bound to the container terminal and on the northern side outward bound from the harbour bridge to the entrance to the Bayswater marina.
Ferries and vessels used by pilot, police, Customs and the harbour master are exempt.
