The Association of Salaried Medical Specialists warns that a new private hospital in Christchurch will take income from other hospitals.
Forte Health will focus on short-stay specialist surgeries and opens early next year.
Association executive director Ian Powell says Forte is defining a niche in the market by targeting less complex surgeries.
Mr Powell says this could be seen by Christchurch's other private hospitals, St George's and Southern Cross, as cherry-picking patients, and he's certain they'll lose revenue.
He says the new hospital's shareholders are all surgeons, but that's common in private hospitals and shouldn't affect the public health sector.
In a statement, St George's Hospital says it's been aware of Forte for a while and is upgrading several of its specialist services to accommodate for the new competition.