Swabs, bandages and drill bits have been left in patients after recent surgeries - and women undergoing gynaecological procedures are particularly affected.
Health boards have reported 63 incidents over the past three years where materials or surgical items were mistakenly left in patients too long.
In one case, a surgical drill bit snapped and a piece was left in a patient's jaw. It was removed later after a more senior surgeon reviewed the case and thought it could cause damage.
Another person suffered repeat infections after a drainage tube was left in their chest for three years.
A Herald survey of adverse event reports for each of the country's 20 DHBs shows about a third of cases happened when swabs, gauze and bandages were left inside women after childbirth or gynaecological procedures. That number may be higher, given many DHBs don't specify what surgery was carried out when listing "retained foreign object" incidents.
Health boards have policies to stop retainment of materials, including making sure every item in theatre is checklisted before and after a surgery. However, in some cases policies weren't in place, or weren't followed. Staff under pressure and poor equipment design were also cited as factors.
At Counties Manukau, a vaginal pack was left in a patient after the pack's "tail" wasn't secured to a leg. Gauze was left in another woman after being taken from a trolley after the final equipment count was done.
Whanganui DHB filed a report in 2015-2016, after a woman who had a routine gynaecological surgery went to ED a few days later because of pain and other symptoms. Six weeks later she was back at ED, in even worse shape. A CT scan showed a surgical clip was blocking a duct connecting the kidney to bladder.
"The patient required further multiple surgeries and unfortunately continues to have problems requiring treatment at another DHB," the report states.
Sue Claridge, coordinator of the Auckland Women's Health Council, said women often bore the brunt of medical mishap.
That was demonstrated by the use of surgical mesh, which was removed from supply for some gynaecological procedures in December 2017, following problems including infection and chronic pain.
The reported cases of retained objects were low, Claridge said, but could be the "tip of the iceberg", given research had shown adverse events were generally underreported.
Doctors were human and mistakes happened, she said. The response was crucial.
"How seriously are they taking patients when they complain something's not right, and what systems and process are being put in place to ensure it doesn't happen again?"
Adverse events are monitored by the Health Quality & Safety Commission, which has circulated documents advising how to guard against mishaps. It also supports simulation training, and promotes the use of a safety checklist, including a count of items such as instruments and sponges.
A spokeswoman for the commission said existing systems were effective in preventing mistakes, and research had proven the effectiveness of the surgical safety checklist.
"International literature suggests only a small portion of adverse events are reported. In New Zealand, DHBs seek to create a culture where staff feel safe to report adverse events, and we are seeing this reflected in the increase in numbers of reported events to the commission."
A change in July 2017 required DHBs to always report "retained foreign object" events, even if there was minimal or no harm. That tweak explained a nearly 60 per cent increase in reported events in 2017/18.
DHBs must review each incident and outline findings and changes made as a result. Most cases involve material such as swabs, but a few incidents involved surgical equipment.
Wellington's Capital & Coast DHB in 2015/16 reported the tip of an instrument used to position the uterus during surgery had come off and been left in a patient.
Drill bits remained in patients in two cases. In one instance at Counties Manukau, the wrong drill guide was used during a jaw operation.
"Excessive bending of the drill led to drill bit snapping," the DHB summarised in its 2015/16 report. "It was initially thought that the drill was flush with the bone, but later review by a senior doctor identified the need to remove retained drill bit due to the potential damage to surrounding tissue.
"Supervision for junior doctors needs to be congruent with level of experience and capability."
Reported cases where after surgery an item such as a swab was mistakenly left in a patient beyond the expected time.
• 2017/18: 27
• 2016/17: 17
• 2015/16: 19
Source: Annual adverse events report by the Health Quality & Safety Commission. A change in 2017 required events be reported, even if minimal or no harm came to the patient.