"He just started yelling at us to drop our stuff, and then he started attacking me. I dropped my bike and I fought back ... he threw me into a fence, but I had my helmet on so I didn't feel anything," Alex said.
The attacker tried to take Alex's bag, but failed and fled with the bike.
Luckily, a friend of the family, Steve Fray, happened to drive past.
Mr Fray said when he looked back in his rear-view mirror he saw the "bigger kid" lunge and begin attacking the pair.
"It didn't stop, he was hitting them as hard as he could," he said. "He was a strong sort of fella."
He felt he couldn't just do nothing, so drove back and got out of his car to try to stop the boy. But the attacker tried to take off with one of the bikes.
Mr Fray said he got back into his car and "herded him like cattle" till the older boy couldn't keep cycling and toppled off.
The bike went part-way under the car, forcing the attacker to cut his losses and run.
Alex called the police.
He said nothing like this had happened to him before. "I was just shocked - I was worried because it was a new bike."
His mother, Nat Sutton, said she was glad her boy was okay.
"I just can't believe that somebody would do that to a kid - from the sound of it, it could have been a very different story for him if he hadn't been wearing his helmet.
"He's physically unharmed, but a bit shocked. He's very disappointed about his bike," she said.
"He's pretty gutted about it - but we'll see what we can do - it was the last of that model, we did a lot of searching for it."
Mrs Sutton said Alex was a lot smaller than the attacker, who she guessed was aged 16 to 18 and was described by witnesses as being Maori or Pacific Islander.
"My boy's tall, but he's very skinny.
"He's never thrown a punch in his life. It's really disappointing that people like that are out there. The cops were fairly quick to respond - they think that Alex was followed from Countdown.
"After the attack he [the attacker] ran into Te Atatu Intermediate," Mrs Sutton said. She believed there were CCTV cameras in both locations.
"We're crossing our fingers that they'll catch him."
As for the bike, Mrs Sutton said it was not in good shape, but it was insured.
"It looks like it will need to be replaced. Looks like it was a very expensive trip to the supermarket for some $2 ginger."
A police spokeswoman confirmed they had been called at 5.10pm on Wednesday, and that a member of the public had witnessed the assault.
"The offender ... was described as male, 17 to 18 years old, 1.83m tall, of medium build with short dark hair."
- Additional reporting: Corazon Miller
