A bowel cancer screening error affected 12,500 more people than earlier revealed, with 30 of them developing cancer.
The Health Ministry said in February a technical glitch involving a pilot screening programme at Waitemata District Health Board meant 2500 people missed out on screening.
It said three people developed the killer cancer as a result and one died.
But new details released today show about 15,000 Waitemata residents missed out - including the original 2500.
Ministry officials said initial analysis showed that more than 30 of those people got bowel cancer.
"As previously, we will undertake clinical reviews to determine if the delay in screening could have made a difference to their outcomes."
Jane O'Hallahan, the Ministry's National Screening Unit clinical director, said they had clearly failed some people "and for that we are sorry".
She emphasized the problems only related to Waitemata residents and the pilot, not the national rollout of screening now under way.
"We have refined and improved our processes for the NBSP [National Bowel Screening Programme] that is currently being rolled out around the country."
In the pilot, tracing people who did not have up-to-date addresses in the National Health Index (NHI) was a challenge, "and at the time of the pilot, our systems for updating records in the bowel screening register from the NHI could have been better".
Health Minister David Clark ordered an independent review in February over the address failure.
Dr O'Hallahan said the minister had been told about the higher numbers and the national rollout will continue.
"Naturally we support the need to affirm public confidence in the national programme and the issues that have come to light with the pilot will be part of that review."
However, Bowel Cancer New Zealand said the Ministry was warned people eligible for a bowel cancer screening pilot were not able to be found.
Mary Bradley told Checkpoint with John Campbell that emails between those running the pilot and the ministry showed there were concerns about tracking people through their addresses.
She said people who did not have updated addresses were often people who desperately needed the screening, but were not getting it.
Ms Bradley said many people would not even know they had missed out on the screening.
Mr Clark declined to be interviewed.
But in a statement he said it was disappointing and concerning that so many people had missed out on being invited to be screened.
He said the latest issue emerged after his decision to call for an independent review of the national bowel cancer screening programme and vindicated that call.
Dr Clark added screening saved lives and it was important people had confidence in the national programme.