A video has emerged online of a man - allegedly a bouncer - punching another man in the face causing him to fall to the ground.
Posted by a Facebook user, the attack was described as "savage" and "out of line".
The video shows two men speaking to each other before one - allegedly a bouncer at a Karangahape Rd bar - strikes the other man early on Sunday morning.
Police are investigating the incident.
"As a result of the video being posted online, Auckland Police are investigating the incident and following lines of inquiry to identify the parties involved, including the victim," Inspector Mark Benefield said.
"We are not immediately aware of the victim's identity however this does not prevent us from investigating and inquiries are underway.
"Police do not condone any form of violence and if anyone has information relating to this incident, please contact Auckland Central Police Station on 09 302 6400 or you can provide information anonymously to Crimestoppers on 0800 555 111," Benefield said.
The woman who posted the video online had left a nearby bar when she saw the incident unfold.
"The bouncer was dragging the victim out of the bar in a headlock," said the woman, who didn't want to be named due to people threatening her after she posted the video.
She said the man being dragged out of the bar was speaking to the alleged bouncer in Tongan.
"He was saying 'we're family', he wasn't offering any violence, but the bouncer wasn't having a bar of it, he was getting all up in his face," the woman said.
She said the pair then separated, and the victim went in for "a handshake hug thing that guys do."
But instead of a handshake he got a punch in the face.
"Then - and this is the ugly thing - he saw I'd got it on camera and so he picked him up and took him back into the club," she said.
She said there were a number of witnesses and the incident had been reported to the police.
She believed the victim was concussed and did not remember the incident, but said he was "ok" now.
It was not the first time the woman had seen this kind of violence in central Auckland.
She said it was about time someone spoke up about it, which was why she posted the video.
"This needs to stop."
The video on Facebook has been viewed 211,000 times and shared 2000 times.
Comments on the video have called the alleged bouncer's actions "unprofessional".
Police could be seen cordoning off a street in the area.