Bosses at the Waikato District Health Board insist bullying is not tolerated in the organisation.

They introduced a rule for the flu season that clinical workers who haven't had a flu jab need to wear a face mask. However the Nurses Organisation said the policy is bullying and is impacting staff morale.

The DHB said four staff members were suspended over the issue, but are now back at work after complying.

Chief executive Dr Nigel Murray said the feedback he's had indicates support for the initiative, and there has been strong uptake of vaccinations.

"We want to work with those who feel that they have been bullied or intimidated. We certainly don't want that. That's not what we've been doing. We consulted with the union and our staff," Dr Murray said.

Murray insisted they have explained the science behind the policy to those who have expressed concern, and most people have moved forward to comply.