The Southern District Health Board is investigating how biopsy samples from two women with signs of possible breast cancer came to be swapped during testing earlier this year.
The mistake led to one of the women having a breast removed when there was no cancer, and the other woman was wrongly told that she was clear.
The testing was carried out at Southern Community Laboratories in Dunedin. Medical director Fitzgerald says a lab worker appears to have swapped the samples over during processing.
Dr Fitzgerald says he has apologised to the woman who had unnecessary surgery and the lab's staff feel terrible that they caused her harm.
He says procedures have now changed to space out the same kinds of biopsy samples so patients can have confidence in the laboratory.
The coordinator of the Auckland women's health council says in other countries, the woman could receive significant financial compensation and ACC should be paying out in this case because an apology is not enough.
Lynda Williams says the mistake comes after a review of the breast screening service in the south and will add to women's fears.