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."
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.