Boaties enjoying balmy conditions in the Bay of Islands have witnessed the unusual sight of a police chase on the water sparked by a launch travelling between dive flags at high speed.

Police wouldn't say exactly how fast the high-powered catamaran was going but it is understood it was clocked at 40 knots, or 74km/h.

The legal limit within 200m of any boat displaying a dive flag is 5 knots (9km/h). The same speed limit applies within 200m of shore and 50m of any swimmer.

The chase started about 8.30am on Saturday near Motukiekie Island.

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The police RIB (grey, on the right) struggles to catch up to the speeding launch in Saturday morning's on-water pursuit. Photo / Stephen Western
The police RIB (grey, on the right) struggles to catch up to the speeding launch in Saturday morning's on-water pursuit. Photo / Stephen Western

Senior Sergeant Peter Robinson, of Mid North police, said the 50-foot (15-metre) launch with four large outboard motors was seen travelling at speed through an area with dive flags.

A police RIB (rigid inflatable boat) gave chase but, given the speed of the launch, it took some time to catch up.

Police had obtained the details of the boat's skipper and owner, which had been passed on to the Northland harbourmaster to decide what action to take.

Charter sailor Stephen Western was on the water on Saturday morning when he was surprised to hear the wail of a siren coupled with the sounds of a police horn, more usually used to signal motorists to pull over.

He believed the initial incident occurred in the passage between Motukiekie and Poroporo Islands but the police boat managed to catch up only at Opunga, as the launch was heading towards Assassination Cove.

''It was crazy to see a police chase in the Bay of Islands, it's not something I've seen before. They were honking, at least 30 knots I'd say," Western said.

Northland harbourmaster Jim Lyle said boaties had to slow down to 5 knots within 200m of a dive flag and had to keep a careful lookout for surfacing divers. Divers also had a part to play by staying close to the boat displaying the dive flag.

''There's a very high risk of fatal injury if a diver surfaces under a fast-moving boat,'' he said.

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The Northland Regional Council, the harbour authority for the Bay of Islands, would act on any information received from police. Possible action ranged from a $200 fine to prosecution through the courts for endangering life.

Saturday's incident is just one of many dealt with by police since the RIB from the Auckland police maritime unit arrived in the Bay on December 29.

The boat is crewed by two skippers from Auckland and a roster of specially trained Northland officers.

Police have also been using their time on the water to assist the Ministry for Primary Industries with enforcing fishing regulations.

The Bay of Islands Coastguard has responded to 18 emergencies since December 10. Photo / File
The Bay of Islands Coastguard has responded to 18 emergencies since December 10. Photo / File

Bay of Islands Coastguard has also been busy so far this summer, responding to 18 incidents and assisting more than 50 people from December 10 until Sunday.

Coastguard volunteer Russ Devin said most call-outs had been for mechanical breakdowns and electrical faults. A crew had also assisted last Friday's search for a diver who went missing off Moturua Island.

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Devin said he wasn't surprised by the speeding launch incident — too often divers didn't use flags, and when they did they were often ignored — but on the whole he was happy with boaties' behaviour this summer.

''We're certainly seeing a lot more people wearing lifejackets,'' he said.

Rescuers have also been busy on the less frequented, but wilder, west coast.

On New Year's Day five people were rescued by locals, Ahipara Fire Brigade and Far North Surf Rescue when their boat flipped at the entrance to Whangape Harbour; and on Friday a man, a woman and a dog had to be towed to safety by Hokianga Coastguard when their motor wouldn't start near the notorious Hokianga bar. In that case all three — including Ripley the ridgeback-cross dog — were wearing lifejackets.