UPDATED 6.45PM: A union says 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.

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 this morning shortly before 11am about an hour after the store opened.

She says as soon as they saw Unite people they locked the doors.

No customers were in the store at the time.

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

She says organisers were yelling at them it was illegal for them not to let the union see its members.

Restaurant Brands denied the union's allegations.

READ MORE: Fast food workers set to strike for fairer pay