Union officials have accused fast food giant Burger King of bullying for threatening to sack a 27-year-old who complained about her work conditions on Facebook.

But Burger King claimed there was a history of disciplinary issues with the employee that the union had ignored in its complaint.

The Dunedin Burger King worker was charged with serious misconduct after writing "Real jobs don't underpay and overwork people like BK does" on her private Facebook wall.

Unite Union national director Mike Treen said he is "appalled" at the charge.

He claimed the fast food giant was panicking about increasing use of social media among its workers.

"It's a comment most New Zealanders know to be true about the fast-food industry in any case," he said.

"We all know these companies are tied at the hip to the minimum wage and workers are frequently run off their feet.

"BK's action is draconian and bullying."

The woman is set to go into a hearing with Burger King officials this morning, Mr Treen said.

Unite had asked for the charge to be withdrawn but were turned down by the company, he said.

A Burger King spokeswoman said the employee had been the subject of disciplinary action before.

The union had chosen to isolate the latest incident out of context, she said.

"This is the latest incident which will be dealt with via the usual disciplinary process. Because we are still within the confidentiality of the disciplinary process, it is not appropriate for us to comment on the details until the matter has been resolved with the employee."

It was unlikely there would be a decision on the employee's future today, she said.