It comes as single mother-of-two Erica Quayle battles ACC for continued cover after she suffered a concussion in a white-water rafting accident in March 2018.
Quayle was sent by ACC to a psychiatrist, psychologist and a neurologist after she failed to fully recover.
ACC suspended the 49-year-old's entitlements, but Quayle won the cover back in May.
Now, another psychiatrist has ruled out post-concussion syndrome and Quayle fears her entitlements will be suspended again.
In 2015, 29 assessments were carried out by some of the Auckland specialists, after which the entitlements of 19 claimants ended. The total spend by ACC was $2.3m.
However, the number of claimant assessments jumped to 1265 just two years later in 2017 and was up to 1458 last year at a cost of $13.9m.
In the same year, ACC paid $138.9m in entitlements to the claimants.
Entitlements include any payments for weekly compensation, social or vocational rehabilitation, elective surgery and other hospital treatments.
ACC pointed out claim decisions were made by ACC and not specialists and sometimes entitlements ended well after the assessment.
"The role of specialists, such as clinical psychiatrists and neuropsychologists, is to provide ACC with impartial, expert medical opinion based on the relevant medical information," a spokeswoman said.
"This is used by ACC as part of the decision-making process when determining cover and entitlements under the Accident Compensation Act 2001."
One of the 254 doctors earned $1.23m over three years for assessing 196 claimants. Of those, 42 were entitlement claim assessments and 23 lost their entitlements after.
The doctor's average assessment cost was $6300 but ACC said money paid to specialists included assessment fees, travel, accommodation and treatment.
Of the 811 claims ACC decided to suspend entitlements on, 27 were taken to review.
Nine reviews were dismissed by the reviewer. Of the nine dismissed reviews, five were appealed to the District Court. One was dismissed and the other four were yet to be heard.
The assessments were completed under four specialities; psychological, clinical psychiatric, neuropsychological or integrated services for sensitive claims.
ACC said a claim could be assessed more than once in a year or by more than one specialist.
Lawyer John Miller, of John Miller Law, whose team is dedicated to challenging ACC decisions, said it was a common theme for ACC to refer claimants with complex or ongoing injuries to specialists for further assessment.
He said the figures obtained by the Herald were concerning.
"It's always a worrying trend where ACC will be trying to get people off instead of using the funds to rehabilitate people.
"Instead they spend millions on reports like this with an aim I think to get people off and put them onto the public health system. Words fail me."
ACC spends about $4.3 billion a year on supporting people with injuries and last financial year it accepted 2 million injury claims.
Miller said the system worked in a way that ground claimants down.
"If you send it to the appropriate psychiatrist or whoever they will say 'well it's not because of physical injuries it's because of other things, you know, bad upbringing and what-have-you'.
"It's a corrosive environment. If you keep having to go through these assessments people just give up. I know people that can't even open ACC letters. It sends them in a panic."
Massey University health sociologist Dr Andrew Dickson has co-authored a paper with Auckland University law professor Jo Manning on the role of external clinical advisers at ACC.
Dickson, who battled ACC after his son was left with cerebral palsy when he was deprived of oxygen at birth, said if the assessor's primary income came from ACC they were "in effect beholden to them".
He said the assessor's reports were "basically unassailable".
"You can't really contest them unless you get your own and they're very expensive and then when you do get your own, ACC are often just going to ignore you anyway.
"In my experience they ignore their obligation to treat these things fairly. They can just privilege the ones they want to.
"And even when you can point to an egregious error, they still don't remove that report from the file or find someone else, or give you cover. There is no blow-back."
ACC said all claims were assessed on case-by-case basis.
"When needed, ACC will contract an external specialist with appropriate expertise to assess complex injuries and provide an independent opinion," a spokeswoman said.
"Our goal through this process is to get unbiased expert opinions to help inform our claim and entitlement decisions.
"It is ACC's statutory role to apply the legal test for causation based on robust evidence."
She said clients had the right to challenge any decision made.
"If a client has any concerns about a decision we've made or the service we've provided, we encourage them to discuss their concerns with us, or they can also make a complaint."
