The country's medical laboratories are being called on to make urgent changes after mix-ups of biopsy samples led to three women having breasts removed when they didn't have cancer.
In a report released on Thursday, the Ministry of Health said that in the past two years, mistakes at labs resulted in five patients receiving major surgery they did not need.
The ministry convened a panel to study how samples of breast and oral tissue from nine patients were mixed up or wrongly diagnosed at hospital and community laboratories.
It meant that three women had breasts removed unnecessarily, one had breast surgery for a tumor that wasn't there, and one had part of her top jaw cut away for no reason.
The ministry says lab quality is high overall, but better processes and automation are needed.
It says the women have been deeply affected and the effects will remain with them for the rest of their lives.
Don Mackie, the ministry's Chief Medical Officer and chairman of the panel, told Radio New Zealand's Checkpoint programme on Thursday the mistakes were human errors of one sort or another.
"They tell a story which, hopefully, will be read by colleagues throughout the sector as rather a sobering one of people who maybe think they're doing the right thing or in a well-intentioned way, but failing to meet the needs of these women who have been damaged during the course of these events."
The Health Ministry says the women's attempts to get ACC cover have been difficult and the Accident Compensation Corporation needs to consider this.
A director of Pathology Associates, Richard Massey, says the process involved in analysing tissue samples is not ideal.
"Unlike much of what happens in other disciplines within pathology, anatomic pathology is little different now from what it was decades ago.
"It is very manual, there are a lot of mechanical steps where people actually have to make decisions about where to put things. Every one of those steps carries a risk."
Auckland Women's Health Council co-ordinator Lynda Williams, says women should be wary of test results.
"This report shows the women of New Zealand cannot yet be confident that there is no risk when they have a mammogram and then have a subsequent biopsy."
The different labs that made the mistakes in five cases have already reviewed and improved how they do things, and all the rest are being given till November to do the same.