A union representing paramedics says WorkSafe "lobbed a grenade" into their collective negotiations with Wellington Free Ambulance.
Wellington Free Ambulance was issued the notice, to do with worker fatigue and staff rosters, after WorkSafe received a complaint from a member of the public.
Amalgamated Workers Union spokesperson Rob Popata said the issue of rosters had been a sticking point in current collective agreement talks, and WorkSafe getting involved had complicated things.
"From our perspective, they got involved in something that they should have let run its course and see if the union and the company could fix something, rather than jumping in - lobbing a grenade into the middle of a pool - and then saying it's just a recommendation.
"Well, they should have kept their noses out at that point and said: 'Let's see if they can reach agreement and if they can't, then we'll step in'."
He said the authority should have involved the unions.
"They were out of line just talking to the employer. You know, when the regulator rings us up, and says we'd like to invite you to a meeting so we can show you the work us and Wellington Free have done, it sends the wrong signal.
"I mean, you've got to talk to the unions involved - they're the workers' voice."
In a statement, a WorkSafe spokesperson said it had found management must look into the risk of fatigue, not just rostering, and union representatives had been spoken to.
The spokesperson said the safety regulator defined workplace assessments as a "targeted and planned examination of a sample of a workplace or work activity to assess the effectiveness of health and safety management".
"As part of this WorkSafe spoke to workers (including union members/representatives), managers, and health and safety representatives prior to an improvement notice being issued.
"WorkSafe has specified Wellington Free Ambulance must look at its management of the risk of fatigue, which is broader than looking at shifts and rosters."
The union will continue talks with Wellington Free Ambulance this coming week.