At Auckland District Health Board, there was a waiting list of 27,200 people for elective surgery last year. Of that number, 4558 patients - nearly 20 per cent - were admitted but left hospital without treatment. Last year up to November, 3822 patients out of 22,346 left untreated.
New Zealand First health spokeswoman Barbara Stewart, who provided the data to the Herald, said the ratio of treated to untreated had "markedly deteriorated" over the past decade.
The Ministry of Health said the figures did not necessarily reflect a health system under pressure. Many patients had their operations postponed because they were unfit for surgery.
"All it means is that the patient is not clinically appropriate for surgery or personally ready for surgery at that particular time," a ministry spokesperson said.
But Ms Stewart said at the very least, the figures were "not the rosy picture painted by the Government but ... a reality many of those who try to get surgery know". She said many of those whose operations were delayed or cancelled were elderly or in pain.
Asked about the figures, Health Minister Jonathan Coleman pointed to the dramatic increase in total operations under National - up 37 per cent since 2008.
"This increase in access is supporting more people to be seen, a key approach in addressing ... demand."
The Government committed a further $27.5 million a year in this year's Budget to meet increased demand for elective surgery. As concerns about "unmet need" grew, the Ministry of Health last year began measuring surgical referrals for the first time.
This meant officials would get a better understanding of the demand for operations, not just the number of people who were approved for surgery.
A New Zealand Medical Journal article published in November showed 36 per cent of people in two DHBs who were considered good candidates for hip or knee operations did not receive surgery.
These patients at Whangarei Base Hospital and Hawkes Bay Regional Hospital met the clinical threshold for treatment but not the financial threshold.
Dr Coleman said patients who did not make waiting lists continued to be monitored by their GPs.