Wait times for urgent, non-urgent and surveillance colonoscopies all progressively worsened last year as Health New Zealand failed to meet demand.
The agency's data shows the total number of publicly-funded colonoscopies dropped by more than 7 percent.
Wellington man Matt Fannin has had annual colonoscopies since he was diagnosed at 18 with the genetic condition Lynch syndrome, which puts him at high risk of developing bowel cancer.
The last time he was overdue for the procedure - about six years ago - he was found to have precancerous polyps.
"So it's a red note on my referral every single time: 'Polyps with high grade dysplasia have been found in the patient 10 weeks overdue'."
His mother, who lives in Feilding, was affected by a pause on surveillance colonoscopies in MidCentral late last year.
When they restarted this year, she was also found to have problematic polyps.
For these reasons, Fannin was on high alert when his usual appointment time came and went.
"I heard nothing, which is unusual - I usually get a call fairly ahead of time to book something in.
"So I contacted my doctor and he said 'Yeah, they're really behind, like six to nine months'.
"I thought, 'That doesn't sound very good. How wide is this problem?'."
He started digging into Health NZ's data and found the problems were nationwide and had got worse over the year.
By January this year, 81 percent of urgent colonoscopies were done within two weeks - the target is 91 percent.
However, only 29 percent of non-urgent cases were done within six weeks - well below the target of 55 percent.
For surveillance colonoscopies like his, 60 percent of patients were more than 12 weeks overdue in January.
The relatively modest target is for 56 percent of people to get their surveillance colonoscopies within 12 weeks of them being due.
"It kind of looks from that data that things are getting progressively worse to the point that it's affecting the urgent referrals so much that they have no choice but to reprioritise and pause things completely."
Matt Fannin finally had his colonoscopy on March 20 - two months late - but felt lucky to be among the 35 percent of Capital and Coast patients getting their surveillance colonoscopies within 12 weeks of being due.
He set up a website 'I'm Overdue' where people could register their wait times.
"People don't complain if it's a little bit late because it's not the most fantastic process," he said wryly.
"But if you leave it long enough, it can be a problem. And it might not be a problem for many years, but it might be the year you leave it that you have a problem."
Health NZ committed to "meeting increased demand"
In a written response to RNZ's questions, Health NZ chief medical officer Helen Stokes-Lampard blamed the longer wait times on the increase in referrals.
"Reasons for growth in referrals include our population growth and a greater awareness of bowel problems.
"Overall demand for colonoscopy is therefore higher than current capacity to meet the demand."
To cut wait times, Te Whatu Ora was recruiting for more staff, running extra clinic sessions and using private outsourcing and locum gastroenterologists, she said.
Health NZ did not answer RNZ's questions regarding the seven percent reduction in colonoscopies last year.
It said it was working on ways to meet increased demand, including FIT tests (faecal blood tests) like those used in the bowel screening programme, to better target those who needed colonoscopy.
Some districts were also using CT scans to examine patients with a low risk of cancer.
Fall in number of colonoscopies
However, Bowel Cancer New Zealand medical advisor Professor Frank Frizelle said he suspected the backlog was mainly the result of fewer colonoscopies being done.
The number of publicly-funded colonoscopies grew by an average of 6.6 percent each year between 2017 and 2023.
"But last year they [Health NZ] didn't purchase so many, and the end result was a 7.2 percent decrease in the number of colonoscopies, which is why there's pressure in this area."
It appeared Health NZ cut back on outsourcing colonoscopies to the private sector, as part of wider-cost cutting, he said.
"Anything that could be deferred was deferred, but those patients didn't go away.
"Delivery of any healthcare has a cost, and what they've decided is to reprioritise their resources away from colonoscopy, which is a mistake."
Every week Professor Frizelle sees patients diagnosed with bowel cancer after funding their own colonoscopies.
"I had three people in one week recently. So yes, it's very common issue with people."
But for many people, going private is not an option.
"They're struggling to get through, and that's particularly the case for those people who already have lots of other reasons why they don't get the best care, including Māori and Pacific people.
"The health system is good if you're able to advocate for yourself and you have a little bit of cash to get around the roadblocks. But if you don't have that, the health system is somewhat challenging."
