Three Northlanders, including an 8-year-old girl, are lucky to be alive after they were picked up by Coastguard when their boat capsized in choppy seas.

The drama unfolded near Rangi Pt in the Hokianga Harbour about midday on Saturday.

The rescue following an emergency call for help from one of those rescued has prompted a call by police for people to check weather reports before venturing out to sea.

A 54-year-old man, a 50-year-old woman and the 8-year-old girl launched their 4.2m fibreglass boat to go scallop dredging near Rangi Pt sometime before 12pm on Saturday.

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On-call Northland police search and rescue incident controller, Detective Ryan Cooper, said a rope got stuck around the crop of the motor in their boat.

"The adults went to the back of the boat to try and unhook the rope in 25 knots wind when water started coming into the boat. Before they knew, the boat flipped over and all ended in the water.

"There were lifejackets on board but only the male was wearing it. The others managed to grab the lifejackets as their boat went over and wore them."

Cooper said the woman had a cellphone and managed to call 111 for help.

He said a police search and rescue team, together with a Coastguard vessel from Hokianga and people on jetski searched for the three locals.

"They spent just over an hour in the water and ended up on a sandbar. Tide conditions were suitable for them to stand on the sandbar and a jetski was able to ferry them to a Coastguard vessel."

All three received treatment for hypothermia after being brought to Opononi.

"They are blimmin' lucky things lined up for them. The lifejackets helped prevent possible drownings. We encourage people to have two forms of communication with them and one needs to be waterproof," Cooper said.

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He said gusting 25 knots was not suitable weather to go out fishing in a fibreglass boat.

Cooper said if people have lifejackets on board, they should make sure they wear them.

Last year, Water Safe New Zealand counted eight fatalities and 18 hospitalisations in water-related accidents in Northland.

Fatality rates average at eight to nine deaths annually over the past 10 years. Northland's rate ranks third highest after Auckland and Bay of Plenty.