The Deputy Health and Disability Commissioner says there has been record growth in the number of complaints received.
Kevin Allan says more than 90-percent of complaints are closed within 12 months, but some files remain open if the matters are complex and cannot be resolved quickly.
He says in the last financial year the commission received 2498 complaints.
That was a 13 percent increase in complaints received every year over the past two years - more than double the six per cent average increase for the three years before that.
The average time for closing a complaint has remained consistent at around three-to-four months.
The former MP and healthcare whistleblower Phillida Bunkle said patients' complaints were slipping through the cracks because the service had been eroded by underfunding.
She says the public remain at risk from bogus doctors through erosion of the service set up to handle patient complaints.
Ms Bunkle's work with Sandra Coney in the 1980s revealed medical malpractice at National Women's Hospital, which resulted in an inquiry and the blueprint for what was previously the Health and Disability Commission.
Ms Bunkle was also instrumental in revealing that health boards in Wellington and Nelson had hired fake psychiatrist Linda Astor in the late 1990s.
She believed it could happen again.
"The performance of the commissioner has been whittled down, and whittled down, and it's been under-funded and their whole role of oversight has been diminished systematically," Ms Bunkle said.
Mr Allan said last year the commission closed 2315 of the total 2498 complaints, which was a 15 per cent increase on the number closed in the previous year.
He said the increase in complaints could be linked to improved access to the complaints process, and better public knowledge of consumer rights.
"Some matters are complex and unable to be resolved quickly," Mr Allan said.
"The HDC has noted an increase in the complexity of complaints we receive, and that complexity reflects a range of factors such as people with more complex conditions, services spread across multiple providers and locations, contested advice, and sometimes other jurisdictions also investigating the same matter."
He said the commission was concerned when complaints took a long time to resolve and it was doing everything it could to minimise that happening.
"We continue to look for ways to manage the increase in the volume of complaints we are receiving but ongoing growth in volume will affect the number of open files we have," Mr Allan said.
"The commission's focus is on protecting and promoting the rights of health and disability consumers. We do this by resolving complaints, holding providers of services to account, and using the findings from our complaints to improve the health and disability sector.
"We are confident that we continue to be an effective watchdog for New Zealanders despite this pressure."