Mammograms of 3850 women taken in Southland have been lost due to an IT fault.
The Southern District Health Board has apologised to patients for the failure, as a result of which mammograms taken between 1 February 2012 and 31 October 2012 being lost.
The images were lost because a server had been wrongly configured and failed to back up copies of the images.
Chief medical officer David Tulloch said on Wednesday there is no change in the outcome from those mammography images; all lost images had been read by doctors and their reports remain in each person's clinical file.
"It's a very significant IT failure, and one that's taken a huge amount of time to sort through, because as we've been working out what the problem was and what has been lost, we also had to continue during the day a normal screening programme. So all this work's been done out of hours - immense personal cost to people's time."
Dr Tulloch says most of the mammograms were done at Invercargill Hospital.
Chief executive officer Carole Heatly says the DHB is working hard to ensure that the failure is not repeated and she urges anyone who needs more information to get in contact.