Issuing warnings to boaties failing to wear lifejackets or travelling too fast near dive flags kept maritime police busy during a two-week deployment to the Bay of Islands.

A police rigid-hulled inflatable boat (RHIB) usually based in Auckland spent 15 days on the water between December 29 and January 13, crewed by skippers from Auckland police and a roster of four specially trained Northland cops.

The most serious incidents were an on-water police chase, complete with lights and sirens, sparked by a catamaran speeding past dive flags at 40 knots (74km/h) on January 5. The offending catamaran was powered by four 300hp outboards so it took the length of the Bay for the police to catch up.

In another incident the police boat had to put itself in the path of another vessel heading straight for a diver who had emerged a long way from his dive flag.

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Tragically, the police boat also helped in the search for a 44-year-old Kaikohe diver who died near Moturoa Island on January 4, then transported members of the National Dive Squad to the site that evening to recover his body.

Coastal master Peter Comer, of the Auckland police Maritime Unit, said the speeding catamaran had passed between two dive flags 300m apart, or about 150m from each.

Northland bylaws prohibit speeds of more than 5 knots (9km/h) within 200m of a dive flag or the shore and 50m from any swimmer.

The speeding skipper, from Auckland, claimed he thought the required distance was 100m and was in a hurry because he was ''late for wakeboarding''.

In the police boat's last eight days on the water Comer said he had spoken to people on 60 vessels, of whom 46 needed education around the rules — mostly for wearing lifejackets, speed or behaviour around dive flags.

''As my focus was on safety and education rather than issuing tickets, I only gave out warnings. If you look at the compliance rate 46 out of 60 it doesn't look great but it needs to be put into perspective. There were hundreds of vessels out there, I just identified those who needed education,'' he said.

A Department of Conservation ranger was on board each day to make sure marine mammal regulations were being observed. Among other things the rules prohibit vessels speeding through pods of dolphins and limit the number of boats interacting with a pod at any time.

The police boat also patrolled popular moorings and DoC campgrounds on Urupukapuka Island.

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■ You can read more about a day in the Bay with maritime police in this Saturday's 48 Hours.