Police want to hear from anyone who may have witnessed anything suspicious at the time of the incident.
The offender has been described as a male Maori aged in his late teens. He has a stocky build and was wearing a black singlet, black jeans and had a white and blue t-shirt around his face.
Detective Sergeant Todd Bartlett said the victim was "understandably quite shaken" and very sore.
"He's suffered some quite serious injuries in the fall and we'd like to identify and locate the offender as soon as possible.
"Takapuna is an area where there's quite a bit of foot traffic and it may be that someone has seen this man acting suspiciously before or after the assault."
Anyone with information is asked to contact Sergeant Bartlett on: 09 477 5261.