Up to 15,000 people in Auckland may have missed out on invitations for a free bowel screening, The Ministry of Health has discovered.
The issue was caused by an IT problem and human error.
The invitation was part of the Ministry's bowel screening pilot programme which invited almost 200,000 Waitemata residents between 2011 and 2017 for a free screening.
Around 117,000 were screened successfully but up to 15,000 people missed out.
National Screening Unit clinical director Dr Jane O'Hallahan said the Ministry took full responsibility for the oversights and would continue to contact those affected to apologise and invite them for screening over the coming months.
"Initial analysis has shown that more than 30 of these people have developed bowel cancer. As previously, we will undertake clinical reviews to determine if the delay in screening could have made a difference to their outcomes."
New Zealand's first screening programme that attempted to enrol all eligible people in the population was an ambitious task for the Ministry team that set up the pilot, she said.
"Tracing people who didn't have up-to-date addresses in the National Health Index (NHI) has been 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.
"We have clearly failed some people and for that we are sorry."
Minister of Health David Clark ordered an independent review into the National Bowel Screening Programme after it was revealed earlier this year that 2500 Waitemata residents missed out on an invitation for bowel screening.
O'Hallahan said the Ministry welcomed the review and was confident its recommendations would lead to ongoing improvements.
Members of the public who had any questions about their eligibility could visit https://www.timetoscreen.nz or call the Bowel Screening National Coordination Centre on 0800 924 432.