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The ACC has forked out close to $675 million in the last decade for patients injured after botched medical treatments, misdiagnosis, post-surgery infections and reactions to medications.
In the last 10 years, the number of treatment injury claims accepted by ACC has increased by 170%, from 3269 accepted claims to 8832 in 2015/16.
Costs have increased by 490%, from $20.8m to $123m over the same period.
And the Government-owned no fault insurer suggests the figures are just the tip of the iceberg - with many treatment injuries bypassing the system.
A treatment injury - which before 2005 was called a "medical misadventure" injury - is a personal injury that has occurred during treatment provided by a registered health professional.
"Treatment' can mean a number of things, including diagnosis, a decision on the treatment to be provided (including a decision not to provide treatment), a failure to provide treatment, or to provide treatment in a timely manner and the failure of any equipment - including implants and prosthetics.
Since 2006/07, $674,768,363 was spent on the claims.
The public money went to compensation for lost earnings, hospital and any ongoing treatment, and rehabilitation support.
Due to factors including demography, and familiarity of health providers and patients with lodging claims, ACC said the statistics could not be taken as an accurate indication of the occurrence of treatment injuries or the quality of care.
This means there is the potential that many treatment injury issues are not lodged as claims.
In December, Minister for ACC Nikki Kaye announced $15m to $20m would be invested over the next five years into preventing treatment injuries.
Initiatives included funding surgical simulation training for operating theatre staff and developing a programme to reduce the number and severity of preventable birth injuries.
But Medical Council chairman and surgeon Andrew Connolly said the majority of treatment injuries were not caused by mistakes, and were simply complications of treatment appropriately given.
"There's a huge difference between a complication and poor care."
Connolly said in surgery there was always a risk of things like discomfort and infection and both constituted treatment injuries.
He said the increase in number of claims showed patients were getting appropriate assistance for their health needs, and the increase in cost could partly be put down to the increasing cost of healthcare.
When the definition of what constituted a treatment injury changed from medical misadventure, the category broadened, making more claims possible, Connolly said.
Medical staff were also encouraged to fill in the claim forms by both ACC and DHBs.
The number of reports to the council from ACC that actually brought into question the competence of a doctor was low, Connolly said.
Last year, the Health Practitioners Disciplinary Tribunal found 40 medical practitioners guilty of misconduct - though grounds for discipline through the tribunal is not limited to malpractice or negligence.
Personal injury lawyer and ACC specialist John Miller said the increase in claims was a result of the 2005 change from "medical misadventure" to "treatment injury", which dropped the need to show fault and opened the category to more claimants.
"It was envisaged at the time that treatment injury would bring in more claims, and it did."
Overseas treatment injuries:
People returning from overseas with injuries caused by medical treatment have cost $1.2 million in public money in the last 10 years.
The ACC has received 345 claims for injuries caused by overseas treatments including boob jobs and other cosmetic surgeries, knee replacements, prostate surgery and dental treatment since 2006.
Of those, 145 were accepted.
Though ACC will not cover the cost of fixing a botched elective surgery, like a wonky breast implant or failed facelift, if a patient becomes unwell as a result of surgery upon return to New Zealand, they could be paid out.
Entitlements a claimant could receive include compensation for lost earnings, hospital treatment and any ongoing treatment, and rehabilitation support.
For an overseas treatment injury claim to be accepted by ACC, the treatment must have been done by, or overseen by a person with qualifications equivalent to that of a New Zealand registered health professional.
The highest number of accepted claims came from reactions to medication (25), followed by breast surgery (9) and hip or knee surgery or replacement (7).
Six claims each were accepted for injuries after cosmetic surgery and injections, five for fracture repair and four each for prostate surgery, anaesthesia, dental treatment and plaster or fibreglass casts.
Though there are no statistics available on Kiwis going overseas specifically for medical treatment (also known as "medical tourism"), the Government warns people considering going offshore for that reason to speak to their doctor first.