Shocking footage of what appears to be an unprovoked attack by a group of men on a nightclub doorman has been released.

The attack on a doorman at popular Auckland club Cassette Nine on Vulcan Lane is being investigated by police.

The doorman was left dazed and bloodied in the attack which happened at the door of the central Auckland nightclub while a bystander who tried to help was knocked out.

Police confirmed they were investigating the assault but could not comment further as their "investigations were continuing".

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"There have been no arrests at this time."

Footage obtained by the Herald on Sunday shows a man in a hooded jacket speaking to the doorman with two others standing alongside him. Two other men can be seen standing further back looking around.

The doorman is punched at least five times before he falls to the ground where the attack continues. He is punched and kicked numerous times in the body and face.

At least five men are involved in the attack with the doorman not throwing a single punch.

A man who tries to intervene is also attacked and knocked to the ground.

It is understood the doorman lost two teeth and was concussed in the attack.

The man who tried to help was knocked unconscious and had his shoulder dislocated.

Cassette Nine operations manager Ruari Hatrick said he was "gutted" with what had happened.

"He's a massive part of our family, he's our protector in a way so it's really shocking for us."

He said the bar had spoken to police as part of the investigation, and been to see the bouncer to see how he was.

"We're just really looking forward to having him back at work."

An independent security consultant who viewed the footage said the attack was just one of an increasing number of serious assaults within the Auckland CBD.

He knew of other seemingly random attacks on the streets and outside bars around the central city.

It was the first he knew of at the bar where the doorman was attacked.

The man said the downtown needed to be a focus for police as "king hits" and serious assaults were a big issue.

"We want the downtown area to be safe for everyone and it needs to be improved with upcoming events such as the America's Cup."

"It's bad for business because people will feel safer elsewhere and it will affect tourism."

A regular patron of Cassette Nine said the incident was the first they had heard of at the bar which was frequented by locals and backpackers.

"It's a great place and there is never any trouble there, the doorman has been working there for as long as I can remember and he's a family guy."

Anyone with information can call Auckland Police on 0800 555 111.