A woman says she had to run screaming and calling for help after a man armed with a "big knife" chased her off the beach in Auckland's Murrays Bay.

The woman spoke to the Herald after four people were injured in a "random" attack at Murrays Bay on Auckland's North Shore.

The woman, who didn't want to be identified, lives close to the beach and had stepped onto it to take her dog for a walk on their regular route when she noticed a man close to the water.

He turned around and began approaching her and the woman at first thought he "wanted to chat".

"I was wearing headphones and went to take the headphones off to talk to him because that's what I thought he wanted," she told the Herald.

However, there was something about the man that put the woman on edge, she said.

Then "he just started running at me".

"And - fight or flight - I turned away and ran the other way and I looked back and he had a big knife, running at me," she said.

"I was screaming, calling out for help because he was chasing me with this big knife."

She said she managed to get a little way ahead of the man and turned away to the left near the sailing club in the area while the man kept on running to Beach Rd where he disappeared.

Passers-by saw the incident and some followed the man at a distance, the woman said.

The woman immediately called police. She said they arrived 10 minutes later and by that time the man had already been apprehended, although she didn't see it happen.

She said the man had "looked a bit dazed" when he approached her.

"My gut was telling me straight away something was up when I first turned to look at him," she said.

"I was a bit on edge, which I think is why I reacted so fast when he started to run."

She said she was now safe and at home but had been "very shaken" by the incident.

Police praise brave strangers who subdued stabbing suspect

Waitematā district commander Superintendent Naila Hassan told reporters that four members of the public were stabbed in the "random" and "isolated" attacks.

Hassan said it would've been an extremely traumatic event for the victims involved.

"This was an isolated incident," she said.

The offender was a local person - and has been apprehended. He had randomly stabbed four members of the public who had suffered moderate injuries.

The members of the public who had acted or helped - including someone using a crutch - had "acted with bravery", Hassan said.

The offender had "quite a large knife", she said.

This was a random attack and there was no indication it was a hate crime, Hassan said.

I really need to acknowledge the bravery of the members of the public who intervened, she said.

"It was a random attack."

The offender was in hospital and had moderate injuries.

From the first call for help to when the offender was apprehended was less than 10 minutes, she said.

On police cordons, Hassan said there would be a highly visible presence in the community, both Murrays and Mairangi bays.

"This was a random attack, there's no indication it could've been prevented."

The person who apprehended the offender with a crutch was not the person who was using the crutch.

Police confirmed people were injured in an incident at Murrays Bay but it was too early to say how they had been hurt.

Earlier: Witness saw man with 20cm knife

A Mairangi Bay resident says he witnessed a man, who looked to be in his 20s holding a 20-centimetre knife, being walked down his street by five members of the public who had apprehended him.

The resident, who wanted to be identified only as Steve, said he was gardening outside his Montrose Terrace home when he saw the armed man being escorted down the road towards the waterfront.

The alleged offender appeared to be in some level of distress, Steve told the Herald.

The people encircling the man didn't appear to know each other and had weapons of their own, he said. As they progressed down the road, the group was telling members of the public to step aside and make way.

Two men struck the knife-wielding man - once in the leg and once in the head - which brought the man to the ground, Steve said.

It was at this point that police stepped in and detained the man, according to Steve.

"The man more or less lay down, I know he burst out in tears, you could hear him crying. I think the realization came in."

Steve was full of praise for the group of men who had surrounded the alleged offender: "They were just so good, they should be commended."

A worker from Montrose Cafe, Bistro and Bar in neighbouring Mairangi Bay says a member of the public used a crutch to trip up the alleged offender, aiding police in his capture.

A member of the public is believed to have used a crutch to stop the alleged offender. Photo / Jed Bradley

A staff member at Cafe BayBay in Beach Rd, Murray Bay says he understands a man stabbed several women, before fleeing to Mairangi Bay.

He said he heard officers caught the offender there.

Another witness said three people had been stabbed, and that three ambulances and three police cars were at the scene.

A person has been arrested after an incident at Murray's Bay in north Auckland. Photo / Daniel Camus

"Emergency services are in attendance at an incident in Murrays Bay where a number of people are understood to have sustained injuries," a police spokeswoman said.

"At this early stage we are still working to confirm the exact number and nature of injuries."

Police have blocked access to the Crows Nest Rise Walkway. Photo / Michael Craig

Police have taken a person into custody, who is assisting them with their inquiries.

Traffic is being diverted and police have asked people to avoid the area.

Police will hold a media briefing at 2.45pm today.

A police officer is seen investigating the scene along Beach Road. Photo / Michael Craig

North Shore MP Simon Watts said he was aware there had been a "stabbing".

"Multiple people have been injured, and my thoughts are with them and their families," he said.

"Thank you to the police, ambulance and first responders for supporting our community today."

East Coast Bays MP Eric Stanford said it was "extremely upsetting to hear about the tragic stabbing".

"My heart goes out to all the victims and their families," the National MP said.