New Zealand has a tendency to normalise and trivialise the issue of bullying at work, according to a director of a workplace agency, who says it needs to stop.

Just under a decade ago New Zealand was ranked as having the second-worst rate of occurrence in the developed world with one in five workers afflicted.

Director of Culture Safe New Zealand, Allan Halse, told TVNZ 1's Breakfast programme a quarter of the agency's clients have been diagnosed with mental health issues, leaving him "horrified".

"We've got probably 25 per cent of our clients and we would have over 80 right now...have actually entered the mental health unit because they have either had mental breakdowns, they are suffering from post traumatic stress disorder or something similar but they are certainly under medication, anti-depressants or sleeping tablets," Mr Halse said.

"It's a lot worse than I ever believed it could be.

"It has horrified me over the last few years."

Mr Halse said a lot of workplace bulling behavior is subtle and takes some time to notice work, such as colleagues trying to undermine others and "setting them up to fail."

"Typically employers will defend bullying managers by saying they're a strong manager but its really easy to see the difference between a strong manager, an authoritative manager, and a workplace bully."

"In Australia they have worked out the cost to the Australian economy as somewhere between $6-36 billion per annum, it's massive.

"From New Zealand's point of view, being second worst in the world, it means we are getting people coming into our country that go into a workplace were bullying has been normalised and they are horrified and can't cope with it."