It's been one of the biggest rescue efforts this summer, and one witness says the outcome could have been much worse.

Emergency services were alerted around 11.30 yesterday morning after several swimmers needed to be pulled from the water at Opoutere Beach, north of Whangamatā.

The incident follows another drowning south of Whangamatā on Tuesday.

Former lifeguard and rescue helicopter worker Tony Brooks was visiting Opoutere Beach with his partner Kathy, a nurse.

They stumbled upon a rescue mission of about 20 people, two of them also doctors, dragging swimmers out of the water, and leapt into action.

Six people were rescued from the sea: one in a serious but stable condition, who was airlifted to Waikato Hospital, four in minor to moderate conditions, and another who later died.

A search-and-rescue effort is still underway for a missing person. It was assisted by a fixed-wing aircraft late yesterday afternoon.

Tony says the group was a family of seven with a bach nearby, who likely encountered bad conditions while swimming.

"While the waves were large, it certainly wouldn't be unfathomable to think you'd just go in for a quick dip and a swim."

Surf Life Saving's operation supervisor Laura Stephens said lifeguards were working with police to search for the missing person. Photo: RNZ / Rayssa Almeida.

But large waves weren't the only danger.

Tony says just out from shore, the beach has a steep drop-off.

"What's happened is they've probably just walked into the water, had a bit of a play around and then just gone over the edge,."

Tony says a local had described to him watching the family get sucked out behind the waves, where they fought to get back ashore.

Without the group's help, it could have been much worse.

"What it was was a collection of some really good people there who were able to assist the situation."

Tony believes the collective specialist skill-set of those involved helped save many of those pulled from the water.

Not only this, he says a surf life-saving rescue board had been left on the beach. It was used by one of the rescuers to bring in some of those stuck in the rip.

Tony, who has never seen this before, praised whoever had left the board there, saying had it not been there, there could have been further tragedy.

Surf Life Saving's national lifesaving manager Andy Kent says the area where the family were rescued is an unpatrolled beach.

He says any unpatrolled beach is not suitable for swimming.

"In swells like this, you shouldn't be taking risks because the swells can be very unpredictable."

The search for the missing swimmer is expected to continue today.

-Finn Blackwell/RNZ.