Armstrong said police were appalled by the attack, which he described as "completely unprovoked".
"This was a particularly violent assault and we are determined to identify the offender involved and hold him to account for this.
"What was also very concerning was the extreme level of violence used, well over and above that needed to overcome the resistance of the victim."
Police have released an image of a man they want to speak to in connection with the incident, captured by a CCTV camera.
Armstrong is confident someone will recognise him - and encourages anyone who does to get in touch.
Police are also wanting to speak to the two other people in the images.
Anyone with relevant information is asked to contact Detective Senior Sergeant Scott Armstrong on (09) 524 1962, or Sergeant Harris on (09) 524 1961 (after 1pm).