Diagnosis delays lead to reading of some southern cancer tests being farmed out.
Breast screening services around New Zealand will help out the Otago-Southland unit which failed to detect 28 women's cancers as quickly as it might have.
Following an internal audit by a radiologist at the Southern District Health Board's breast screening unit, the Ministry of Health has moved to reinforce the unit's safety.
The ministry and DHB have begun an urgent investigation because of the concerns raised by the audit, although the auditing method, because it was based on known cancer cases, has an inherent bias.
The re-reading of mammograms is likely to have been more sensitive to cancer signs than the initial reading would have been.
Asked if the Government's breast screening services were safe, the ministry's chief medical officer, Dr Don Mackie said: "I'm comfortable the national [programme] is safe.
"We don't have enough information specifically about [the Southern DHB] at the moment but we are working to get that. Some of the steps we've put in maintain its safety now."
Some of the reading of southern women's mammograms will be farmed out indefinitely to radiologists at the other seven providers in the BreastScreen Aotearoa national programme.
All screening mammograms are read twice, and if there is disagreement about the findings, a third read is done.
A Dunedin radiologist in the southern unit who had some concerns about the programme conducted the audit of the mammograms of women diagnosed with cancer from 2007 to 2010.
"She found a number of them which she thought probably should have been brought back for recall on the basis of an earlier mammogram," Dr Mackie said.
The 28 women were not recalled, so the diagnosis of their cancer might have occurred later than if they had been recalled.
The principle of early detection underlies the screening programme, but because cancers vary, it is unclear what effect the delays may have had on the 28 women.
The programme offers a free mammogram every two years to women aged 45 to 69. If an abnormality is detected, a woman can be recalled for another mammogram and possibly an ultrasound scan and needle biopsy.
The audit radiologist concluded that nine of the women should definitely have been recalled for further investigation, although another radiologist who reviewed the images put the number at seven.
They agreed that a further 19 - the second radiologist said 21 - should also have been recalled, although these women's images conveyed a less clear-cut suspicion of cancer.
Dr Mackie said the DHB was contacting the women to explain what had happened. When asked if a radiologist might not have read the women's mammograms correctly, Dr Mackie said, "At this stage, we are thinking it's more in terms of the system rather than any individual."By Martin Johnston Email Martin