Police said it was a cowardly attack on a well-meaning person trying to help someone he thought was in need.
The attacker is described as being around 1.8m tall, aged between 25 and 35, of stocky build, brown-skinned, and with a bushy hairstyle.
His vehicle was described as a black or dark-coloured four-door saloon.
He may have been with others not seen by the victim, or may have tried to flag down others before or after the assault.
The offender may also have stopped in the Queenstown, Arrowtown or Wanaka area before or after the assault.
Anyone with information about the attack, which happened about 10.30pm, or the offender, is asked to contact police on 105, quoting file number 210327/4376.
Information can also be provided via Crimestoppers on 0800 555 111.