The senior doctors' union claims a pilot programme at Waitakere Hospital in Auckland is causing considerable frustration for workers.
Some operations which would normally be contracted to the private sector are being carried out by Waitakere District Health Board hospitals instead.
The board says patients are recovering faster and productivity has shown a 25% increase.
But the Association of Salaried Medical Specialists says the pilot is causing friction, with surgeons getting up to $10,000 extra a day for the work on top of normal duties.
The union's executive director, Ian Powell, says the programme is perceived as unfair because some specialists see colleagues earning a lot more.
The DHB concedes the programme is controversial but says it needs to be experimental.