The Transport Accident Investigation Commission (TAIC) has launched an inquiry into a boating tragedy which left three men dead in Auckland's Manukau Harbour yesterday.

A team aim to recover any wreckage of the boat and to scour videos or photos of the incident.

A man, 23, was airlifted to Auckland Hospital and three bodies were recovered after a boat overturned near Manukau Heads, near the notoriously dangerous bar, yesterday afternoon.

Police said the three victims were men aged 54, 61 and 70. The fourth man is in moderate condition.

Police said they are continuing to work with Maritime New Zealand to "understand the circumstances leading to the deaths", and are making inquiries on behalf of the coroner.

The TAIC is appealing for witnesses who saw, photographed or videoed the boating accident. It's understood other boaties found three people dead in the water and rescued the fourth.

"Getting the facts straight is vital, so we're keen to hear as soon as possible from people who saw the accident or observed the boat at any time in its journey," said Harald Hendel, Chief Investigator of Accidents.

"Details of the boat involved are not known at this stage and this is where witness accounts may help, particularly if you have photographs or videos."

A dedicated team will be travelling to Auckland tomorrow to investigate. They want to interview witnesses and see any photos or videos taken of the incident. Investigators will also seek to recover any wreckage.

"We're interested in what people have to say, of course, particularly the survivor, but also what the families and friends of the crew knew about their plans for the trip," said Hendel.

"We're keen to find out more about the boat, its individual and type history, performance, maintenance, equipment and design."

Witnesses can email TAIC at [email protected].

The deaths have been described as the worst tragedy on that body of water in many years by Coastguard.

Chief executive Callum Gillespie is urging Auckland boaties to be prepared, as hundreds of vessels - numbers matching peak summer levels - were seen out on the water yesterday.

Five people have died on Manukau Harbour in the past week. A diver and kayaker have also lost their lives since alert level restrictions eased in Auckland, allowing residents to participate in water sports.

"Three people dead in a single event is one of the worst tragedies in Manukau Harbour in a number of years," said Gillespie.

Coastguard teams from Papakura and Waiuku were deployed and searched "unsuccessfully" for the overturned boat.

Yesterday police said they were investigating how the vessel ran into trouble.

Gillespie said the boat involved did not make a bar crossing report with Coastguard, but they did receive in excess of 200 bar crossing reports from the Manukau Harbour yesterday alone.

It was Coastguard's own "Super Saturday", with more than 1700 trip reports and 29 incidents, he said.

"That's a big day in the summer, never mind in October."

Most incidents were non life-threatening, involving flat batteries, people running out of fuel and mechanical failures.

But they showed many were not prepared to head out on the water.

"Understandably people have been locked up in the winter, through this lockdown period and their enthusiasm to get out ... is not unfortunately being matched by preparedness," said Gillespie.

He urged boaties to remember to check the weather conditions, pack lifejackets and two forms of communication, and to put in a trip report or bar crossing report with Coastguard.

"It's about making sure whether you're diving or kayaking or in a boat that your equipment is good and well checked and it hasn't rusted or become ineffective through the winter.

"If you leave your fuel sitting in your engine over winter don't be surprised if it doesn't work."

It's unsafe to cross the bar at Manukau Harbour at low tide, Gillespie said, and he advised boaties to check the tide.

"Wear life jackets because when you're in the water it's too late to put the lifejacket on."

Coastguard are preparing for another bumper summer season with New Zealand's borders remaining closed.

Gillespie said boat trailer manufacturers are already being "worked off their feet".

"I think we're going to see a repeat of last year. It's going to be very busy.

"We are getting prepared for it and we would ask that Kiwi boaties do their bit by getting ready for it too."

The Manukau bar is notoriously dangerous and has led to a number of rescues, including two earlier this year.

Boaties can plan their trip here.

Safety tips for crossing the Manukau Bar

• Always check the swell, tide and wind conditions – every crossing of the Manukau Bar is different because of these factors.
• Always try and cross the bar during daylight hours.
• Make sure that everyone on board is wearing a lifejacket and is awake.
• Crossing at high tide is best (but always take the weather and swell into consideration too).
• Ensure that you are carrying two forms of communication that work even when wet.
• Log a bar crossing report with Coastguard by calling *500 on your mobile or contacting Coastguard radio from your VHF. Once over the bar safely close your bar crossing report.
• If in doubt, don't go out!

Source: Coastguard