The Association of Salaried Medical Specialists says other options should have been looked at before a trial using health workers from the United States was funded.
About $500,000 will be spent paying for two US physician assistants to work in Counties-Manukau over the next two years.
Physician assistants are widely used in the United States, but the union says what make sense in a large country, may not work in New Zealand.
Executive director Ian Powell says thought should have been given to whether existing groups, such as nurses, could have their roles expanded, before considering creating new occupational groups.
He says extensive work must be done to ensure the trial is not a waste of money.