When Cory Sonntag fell off his bike he thought the man coming towards him was going to help.

Instead, the stranger threatened the 11-year-old with a knife - and said he'd kill him if he didn't hand over the much loved Christmas present.

Cory's mother Lynn Sonntag now wants to warn other parents after her son had his bike stolen at knifepoint in broad daylight.

The Glenfield student was riding his bike at the Birkenhead Pump Track in Birkenhead War Memorial Park around 11.30am when he fell on Wednesday.

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As he was getting up, he saw a man approach.

"I thought he was coming to help," Cory said.

"But he grabbed my bike and told me to give it to him. I said no, and he said 'Give me your bike or I'll kill you'.

Cory Sonntag with his mother Lynn at the Birkenhead skate park. A man threatened to kill him with a knife before stealing his bike. Photo / Jason Oxenham
Cory Sonntag with his mother Lynn at the Birkenhead skate park. A man threatened to kill him with a knife before stealing his bike. Photo / Jason Oxenham

"Then I saw he had a knife, and I stepped back."

The man, who is described as 25 or older with brown skin, a black beard and hair, wearing black clothes and a black and white bandanna around his neck, fled with the bike into the bushes.

There were no witnesses to the attack, but a worker at the nearby Birkenhead Pool alerted police. A police dog and helicopter were sent but the man was not found.

"I want people to know, because they don't realise there is that danger at the bike park," mother Lynn said.

"People should be aware that there's someone out there doing this to children."

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She is angry that despite signs at the park warning of camera surveillance, the cameras were not operational. The park has been open a year and Kaipatiki local board deputy chairman John Gillon said he would investigate the situation.

Cory says he is now extra wary when venturing out in public.

The unusual black and yellow Haro bike was just six months old, a Christmas present from Cory's nana, and it was a daily means of transport.

Lynn said they have been overwhelmed by the response to her Facebook post about the attack, with offers of replacement bikes and even a trip on a Harley Davidson.