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The robbery took place last Saturday around 8.30pm when four men entered the Ti Rakau Liquor store in Burswood.
One man picked up some alcohol and tried to walk out without paying, police said.
When confronted by the store's lone employee, another man from the group attacked him.
All four men in the group then set on the employee, punching and kicking him before he was hit on the head with a bottle of spirits so hard the bottle shattered.
The four men then left the store, taking with them some Corona beers and a few bottles of spirits, police said.
The employee was knocked unconscious during the assault and was taken to hospital. He has since been released.
Counties Manukau Police Detective Senior Sergeant Darrell Harpur said the extreme violence used against the defenceless employee was concerning.
Anyone with information relating to the incident were asked to contact Detective James Ralph or Detective Sergeant John Love at Counties Manukau East
Police on 09 250 2800. Information can also be provided anonymously through the Crimestoppers line on 0800 555 111.