Some Burger King Linwood staff took time off because of a group of youths' threatening behaviour.

Fast food chain Burger King says it has trespassed a group of youths from every one of its Christchurch restaurants after they terrorised staff and customers at its Linwood store.

The teenagers, thought to be aged 14 to 17, allegedly took knives into the Linwood Ave restaurant, cut up furniture, stole drinks, threatened violence against staff and assaulted a customer.

Some staff were left too scared to work, a security guard was now there at night and police were increasing patrols in the area.

The group, usually involving three to six youths, first started causing trouble at the Burger King on April 3 and had turned up at least once a day since, a union organiser said. Nearby businesses reported similar issues.

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A dairy owner said he and his wife had considered quitting because of almost daily thefts by teenagers. One recent day a group stole 10 energy drinks, then came back two hours later and stole four more. The man, who asked not to be named, said he bought security cameras, called police and threatened to trespass the teens, but they "don't care".

Police confirmed they were called to Burger King on April 3 after youths reportedly assaulted a man who refused to give them tobacco. He was taken to hospital where he received treatment for facial injuries.

A spokesman for Burger King's New Zealand franchisee, Antares Restaurant Group, said he was unaware of a customer being attacked, but said it had ensured a security guard was in place in the evenings and "our most senior regional managers" were in the restaurant during other operating hours.

Security footage was provided to police. The youths were identified and subsequently trespassed from "all our Christchurch restaurants", the spokesman said.

"The safety of our team and customers alike is a significant priority and we took steps immediately once senior management were made aware of the issue."

Unite Union, was concerned not enough was being done.

Organiser Nicole Campbell said the youths caused "havoc" in the restaurant. She said on Monday a staff member had a drink thrown at them when they asked the youths to leave. The group then allegedly hit some windows and kicked the front door.

Campbell said two staff took a day off this week because they were too scared to work, one person had a panic attack, while others downplayed the threats as "what you expect from Linwood".

"They shouldn't feel that way. They should be able to feel they're in a safe work environment, but they clearly don't."

Having an evening security guard was not enough, as the youths went in during the day, she said.

Canterbury Metro Commander Superintendent Lane Todd said police had received several reports since April 1 about youths threatening staff and customers and vandalising property.

Since then, police had responded to reports from the restaurant on three occasions. Officers were still investigating the alleged assault of the man on April 3, he said.

On Saturday, police responded after two youths were reported to have broken a sign in the restaurant. The two male teenagers were spoken to and taken home.

Police were "aware of the ongoing concerns over this group of youths and the harm they are causing".

An area prevention team had been assigned to visit the restaurant and work with management to identify "prevention opportunities", while extra patrols would be deployed in the area, Todd said.