He was arrested early this afternoon and has been charged with aggravated wounding with intent to cause grievous bodily harm and assault with intent to rob.
He will stay in police custody overnight and appear in the North Shore District Court tomorrow morning.
Earlier this afternoon, the family of the bashed mum-of-six issued a public statement, and say they are "shocked and upset" about what happened to her.
Ms Knight was hit on the back of the head when she intervened in a bag snatch attempt at the Northcote Countdown supermarket yesterday. She fell and hit her head, fracturing her skull and suffering a brain bleed. She underwent emergency surgery last night.
"Lucy has undergone surgery and is recovering in hospital and we are here with her," her husband Peter Thomas said on behalf of his family.
"At the moment our main focus is Lucy and simply being with her. Our children are in good hands. We thank our family and friends for their love and support, and we'd also like to thank the many others who have offered their concern and best wishes."
Mr Thomas asked for privacy while Ms Knight recovers.
A Givealittle fundraising page has been set up by friends of Ms Knight.
Ms Knight's family have been inundated with offers of practical and financial assistance, and her friends set up the page on Givealittle to allow members of the public to donate.
Police: Huge response from public
More than 120,000 people have viewed photos of the youth who allegedly attacked her, after CCTV footage was posted on the Waitemata Police Facebook page.
The post has also been shared by a further 2000 and police want to thank the public for their assistance.
The investigation into Ms Knight's assault is ongoing with police launching Operation Vault in a bid to find the youth responsible.
Detective Senior Sergeant Stan Brown said the Operation Vault team had received a number of calls following the publication of the photos and they were working through the information and following several lines of inquiry.
The Asian woman who Ms Knight stepped in to help during the attempted bag snatch has also been identified after her family contacted police last night.
Mr Brown said she would be spoken to today with the help of an interpreter.
"Police would like to thank the media and the public for their assistance to date, including everyone who has shared the photos of the offender," Mr Brown said.
"The post on the Waitemata Police Facebook page has been viewed a record-breaking 120 256 times since it was posted last night and nearly 2000 people have shared it. This is a prime example of safer communities together and we thank both the media and the public for their assistance so far."
Sightings of car sought
Police are appealing for sightings of a vehicle the alleged offender was seen getting into after the attack.
"He is believed to have gotten into a white station wagon with tinted windows," Mr Brown said in a statement this morning.
"The vehicle is believed to have been driven by a woman and police would also like to hear from anyone who may have seen a vehicle fitting this description in the Northcote area around 1.40pm."
Husband: 'She's had a peaceful night'
Ms Knight, the wife of Epsom Girls' Grammar teacher Peter Thomas, had her two youngest children aged 2 and 4 with her when she was injured.
Mr Thomas has updated friends and family on Facebook after spending most of the night at his wife's hospital bedside.
"Lucy has had a peaceful night after surgery yesterday," he wrote this morning.
"Right now she is sleeping reasonably comfortably. We hope to know more over the coming days. Thank you all for your love and support."
Messages of support flood in
Ms Knight is an active member of the Facebook group Mums on Top and messages of support were flowing on the page this morning.
"Our thoughts are with fellow MoT and mum of six Lucy Knight after she suffered serious head injuries when she tried to stop a bag-snatch at an Auckland supermarket yesterday," a page administrator wrote.
Mandy Phipps-Green responded: "What a courageous woman to help the (other) lady. I sincerely hope she will be ok."
"Dreadful that she's been injured, and I know that it's probably no comfort to her family, but she did an incredible and brave thing to help another person in trouble, without thought to herself," she wrote.
Stella Schouten said she was "horrified" to read about Ms Knight's ordeal.
"Firstly, how terrible to snatch a bag from an old lady and then to even consider what they did... I hope Lucy pulls thru okay and her family has lots of support around them at this hard time."
Others on Facebook said Ms Knight was a hero and were saddened to hear she had been hurt while trying to help someone in trouble.
Witnesses describe how the incident unfolded
The incident unfolded as the youth tried to steal a woman's bag as she entered the Countdown Supermarket at Northcote on the North Shore about 1.40pm yesterday. As he did, Ms Knight, stepped in to try to stop him.
He hit her on the back of the head with his hand, causing her to fall and fracture her skull, as he ran away empty-handed.
Michael Dudley, 21, works at a takeaway shop nearby and was there when the drama unfolded.
"I was coming out of Countdown and I heard a struggle behind me," he told the Herald last night.
"I turned around to see a swinging arm. Then the lady fell, she went down really fast and hit her head on the concrete. I saw the young kid take off down the carpark and I started to chase him."
The youth got into a car that Mr Dudley said was waiting for him. He saw a woman in the driver's seat.
"It was running and the driver was already starting to turn around. I started banging on the windows, I couldn't see in because the windows were tinted black. That's when she [the driver] ran over my foot."
The car, described as a white stationwagon or family wagon, likely a Nissan or Subaru Forester, took off at speed and Mr Dudley could not see the number plate.
He ran back to Ms Knight, who was surrounded by members of the public.
"I checked on her, started talking to her. She wasn't really saying much, just mumbling," he said.
"I asked if she had someone who lived close by, like her mum. She answered 'yes'. I just asked her questions and she mumbled 'yes'."
Mr Dudley, who gave a formal statement to police last night, said the woman's mother was listed in her cellphone under 'Mum'. She was called and came straight to the carpark.
By that stage police and paramedics were there and the 43-year-old was rushed to hospital. Her mother followed with the children.
It was unclear last night whether her head injury was life-threatening, but police said it was "serious" and the surgery took about three hours.
Alleged offender's description
Police released images of a youth fitting the description of the alleged offender, captured on CCTV cameras around Countdown. He was filmed in the area prior to the incident and then again running away afterwards.
Witnesses described him as a young Maori or Pacific Islander aged about 15 to 16 years old.
He was wearing black clothing and a baseball cap.
"This woman was acting as a good Samaritan," said Detective Senior Sergeant Stan Brown.
"She's sustained a serious injury in front of her own children and we're determined to identify and apprehend the offender as soon as possible. If you were there, please call us."
Police were also appealing to the woman whose bag the youth tried to grab to contact them. She left the scene before police arrived.
Countdown: We'll help as much as we can
Countdown spokeswoman Kate Porter told the Herald: "It happened extremely fast and the team immediately called the police and ambulance and helped to support the two customers involved and their children.
"We've worked with the police to provide footage and as much information as we can, and will continue to do so. This is an unprecedented situation involving a customer who was simply trying to help do the right thing."
Ms Porter said the company would review what had happened during the incident. "Our focus is helping the police with their inquiries so they find the offender."
She added: "Our thoughts are with the victim and her family at this difficult time."
Xiuqin Shen, owner of the Busy Horse Takeaways next to Countdown, said she'd witnessed a bag-snatching incident in front of her shop a few months ago.
"A boy came and [snatched] the woman's bag. There's always something happening," she said.
Her landlord had installed cameras at the front and back of the shop for extra security.
However, police said they were not aware of any similar incidents in the area recently.
"But like any serious crime, police will be looking at the possibility of any other related crimes," said Mr Brown.
Anyone with information on the incident was asked to call North Shore Police on 09 477 5261, or Crimestoppers on 0800 555 111.
Good Samaritan attacks
It was not the first attack on a good Samaritan in Auckland.
In 2008 father-of-three Austin Hemmings was fatally attacked as he stepped in to help a woman who was being accosted by another man in the CBD. Mr Hemmings was stabbed to death as he tried to help the woman get away from the offender in Mills Lane, off Albert St.
Also in 2008, a woman was killed when a bag snatch at a shopping centre went wrong. Joanne Wang was run down in front of her young son by a group of men in a stolen 4WD in the carpark at the Manukau Westfield shopping centre.
By Anna Leask of the New Zealand Herald
