The union for workers at The Warehouse says it fears the company has answered to claims of bullying with "veiled threat" towards those who spoke out.

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First Union says a Warehouse email about complaints laid against a manager at one branch contains a veiled threat.

The email was sent to staff after reports of bullying against a manager at one branch of the retail giant were made public.

The Warehouse said in a statement if teams had bullying or other accusations, they should use the appropriate channels to let them know.

More than 20 staff had made complaints, and reported being yelled at, picked on for their age, ignored, and given impossible workloads.

Following the reports, The Warehouse sent an email to staff assuring them the company did not tolerate bullying, and took swift action on any allegations.

It said the the matter had been investigated and claims of bullying were not substantiated.

The email also said The Warehouse would be investigating the conduct of those who had spoken to media.

The union that represented staff was horrified by the email, First Union national co-ordinator for The Warehouse Kate Davis said.

She said parts of it were untrue.

"They said 'we've responded swiftly'. They didn't, they've had literally years of complaints that they did not respond to. They only responded when we collated all of the complaints into one place, so they didn't respond swiftly."

The Warehouse had also said it had interviewed all the people involved, but Ms Davis said they had not.

She feared the comment at the end of the email discussing the conduct of those who had spoken out was a "veiled threat".

In a statement to RNZ, The Warehouse said the email was sent to staff in the hopes of trying to understand why the union and some team members took the matter to the media. It said teams should let them know through the appropriate channels if they had any accusations.

There was a confidential email address where team members could raise anything confidentially.