Police have confirmed multiple fatalities in a boating accident in the Kaipara Harbour.
A statement from police said "a number of people" were dead after a boat got into difficulty in the harbour on Saturday afternoon.
A Kaipara fishing industry source confirmed the boat involved in the tragedy was Francie, a 12m fishing charter that can carry a maximum 20 passengers and two crew.
At this stage police were unable to confirm the death toll. They are still working to establish the identities of those onboard.
It's believed six people have been rescued and are receiving medical treatment, but police were unable to confirm numbers.
"The rescue operation continues this evening and is fully resourced, with the assistance of the Coastguard, St John, Surf Life Saving and Victim Support."
A North Shore Hospital spokeswoman confirmed three people had been taken to the hospital. They were in a stable condition.
An ambulance spokeswoman confirmed two people were taken to the hospital by ambulance. It was not know how the third was transported.
An Auckland City Hospital spokeswoman said they had not received any patients from the incident.
It's understood five people are missing following the capsize, potentially making the incident the worst maritime tragedy since the Easy Rider capsized in Foveaux Strait in 2012, killing eight.
"It shouldn't have been out there"
The former skipper of a the Francie has tonight described it as an "excellent little boat", but said it should not have been on the harbour today.
"It shouldn't have been out there. It was too rough ... it's a terrible thing," Rod Bridge said.
He also runs a charter business, which he declined to name, but said he had disappointed his customers by calling off today's outing because of the conditions.
"I stayed home, I'm not stupid .. the wind was coming up and the swell was coming up. You wouldn't go out over the bar."
One customer had called him this evening, after hearing of the tragedy involving the Francie.
"He said 'thanks for making the right decision'."
Bridge owned the Francie, which is made of steel, for two or three years. He bought it about four or five years ago, when it was around eight years old.
"It was a flaming good little boat. That's why I bought it."
He believed the boat's owner, Bill McNatty, was the only skipper of the Francie.
Charter owner: Sad to miss one of our fellow skippers
The owner of a charter fishing company that also takes groups out on the Kaipara Harbour said it was a sad situation for all.
The man, who asked not to be identified, said he knew the skipper of the Francie and was just waiting to see whether or not he was one of those who had died in the accident.
"It's one of our friends so we don't want to give out too much information.
"It's a bit sad to miss one of our fellow skippers - well, we don't know yet. But it's a bit sad to hear what's happened.
"Just [feeling] for the families.''
The man said he happened to be out with a group in the Harbour today, but had fished inside the Harbour as the group did not want to go out further.
"You just got to be careful out there when you're going out to sea - not take too many risks."
He said conditions could be very rough and those heading out had to really be experienced and keep an eye on the weather.
Wayne Bollond was having a drink at Helensville's grand Hotel as he anxiously awaited news of his old mate Bill McNatty and the others on board the Francie.
Wayne worked for McNatty for about 18 months and said he was an extremely experienced skipper who'd made that same trip countless times.
He said the Francie was a purpose-built steel charter boat that would have been fine in marginal conditions.
A typical trip would involve catching a few kahawai at Potu Bank and then going for snapper at the 'Graveyard' - a mussel bed on the inside of the bar at North Kaipara head.
"It's a very, very safe boat - I have total respect for it. I believe something else would have happened and that's when people lose their lives. That's how safe you've got to be out there and that's how safe we are. We're professionals."
He said there was a "confused sea" over the bar that required skippers to notify Coastguard prior to entering it and upon leaving it.
He described the incident as a "tragedy".
"We will be grateful"
A spokesman for Maritime NZ said it was not appropriate for the organisation to comment about the situation, given the police response and investigation was still underway.
However, he encouraged anyone who may have information they would like to provide to authorities to make contact.
"If people have maritime information they believe is important or have concerns they wish to raise, then please contact Maritime NZ.
"We will be grateful for their call.''
Police to boaties: "steer clear"
Three helicopters and a plane continue to search for people in the water.
Police have also appealed for boaties and pilots of aircraft not involved in the search to "steer clear" of the area.
"Emergency services require a safe working space around South Head," police said.
An online booking service for Francie states: "Enjoy a day's fishing for any of the predominant Kaipara fish species - kahawai, snapper and gurnard and in season trevally, maumau and kingfish".
Daily boat hire costs from $700-$800.
In 2012 the boat was involved in the rescue of a ketch cast adrift after its sails blew out and it had engine problems.
A story in the Kaipara Lifestyler quoted then Francie skipper Bridge as saying: "Another half an hour and that would have been it - they would have foundered in heavy seas on the bar."
A rescue chopper and Coastguard were also involved in the 2012 water rescue.
Today, emergency services were alerted just after 3.30pm about an overdue vessel off South Head.
Earlier, a spokeswoman for the Auckland Westpac Rescue Helicopter said they had been called to an accident that left "multiple people in the water".
Lifeguards said 11 people were in the water after a boat flipped over.
Police said they were co-ordinating a search and rescue operation in Kaipara Harbour with Coastguard.
The Muriwai Surf Lifesaving Club was being used as a base for the emergency operation.
The Transport Accident Investigation Commission confirmed it had opened an investigation into what it described as a "marine accident".
"It is tracking the continuing search and rescue operation, and deploying two investigators to the scene who are expected to arrive in the morning," it said.
"The Commission's inquiry will seek to explain the circumstances and causes of the accident, with a view to helping prevent similar accidents in the future. The Commission is unable to confirm further details of this afternoon's events while the search and rescue operation continues."
MetService said the mean wind speed in the area, recorded at 5pm, was 28km/h gusting 50km/h.
Meteorologist Tom Adams said that at 6pm, that had changed to 31km/h and gusting 48km/h.
The air temperature in the area is 18 to 19C and a few light showers are expected.
The Kaipara Harbour features one of the toughest bar crossings in New Zealand for boaties to encounter.
"They are common around the country in areas such as harbours and rivers," an education page on Coastguard Northern Region's Facebook page says.
"If you go boating and want to get out into open seas you usually have to navigate them and they can also be extremely dangerous. They are called Bars, such as can be found at Manukau, Kaipara, Raglan and many other areas in New Zealand and around the world.
"Many experienced boaties have got into difficulty crossing a bar and each year, boats are damaged and people are killed or injured when attempts to navigate bar crossings go wrong."
Kaipara Harbour's bar has resulted in as many as 110 shipwrecks.
• Anyone who has concerns about the welfare of relatives in relation to this incident is asked to attend Helensville Police Station or call 111 for assistance.