Carl's Jr staff were locked inside a Christchurch store for at least half an hour this morning in an attempt to stop them striking, their union says.

Restaurant Brands staff at KFC, Carl's Jr and Starbucks stores around the country are striking today after contract negotiations broke down.

About half the 4000-strong work force was planning to strike.

Unite Union organiser Nicole Campbell said she arrived at the Bush Inn branch of Carl's Jr shortly before 11am, about an hour after the store opened.

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"As soon as they saw us they locked the doors."

No customers were in the store at the time.

KIDNAPPING - Restaurant Brands have taken union members hostage. Carl's Jr Bush Inn in Chch has locked it's workers IN so they can't join the strike.

Posted by Joe Carolan on Friday, 21 April 2017

A video posted live to Facebook shows someone outside the store trying to open the locked door and staff can be seen behind the counter inside.

It was at least half an hour until the manager unlocked the doors again, Campbell said.

"We were yelling at them it was illegal for them not to let us see our members."

Comments on Facebook pointed out locking staff in store could be seen as a health and safety hazard if a fire broke out.

Restaurant Brands has been sought for comment.