The reports are then used to judge what a claimant should - or shouldn't - be entitled to.
The insider told the programme specialists who don't provide the desired findings are either "guided" by ACC to provide the preferred assessments, or they are not used at all.
South Auckland lawyer Phil Schmidt has been fighting for ACC claimants for 10 years, and backed the claim that there were "favoured assessors"
"Those assessors become favourites because they simply are more likely to tell the case manager what they want to hear," Mr Schmidt told 60 Minutes.
"Which more often than not is this person has the capacity to work."
60 Minutes reported that specialists, many from small towns, were being flown around the country to perform assessments in other regions.
In one particular case, a Christchurch doctor pocketed $1.2-1.4 million over a two period, assessing between 1200 and 1600 clients around the country.
"If you have someone paying you huge amounts of money I don't believe you can be objective about writing reports back," Mr Schmidt said. "Now, they may believe they're objective, I think you can tell yourself that - if it is in your financial interests to do so."
Mr Schmidt believed "hundreds or thousands" of reports conducted by these specialists would be wrong.
"It's disastrous, because these people can't work, many of them have very serious injuries and quite high levels of pain. What that means is people can't pay their mortgages and they lose their houses. What that means is families are under huge financial stress and the marriages can break up."
However neurologist Victor du Plessis, a medical assessor for ACC who has been flown around the country for assessments, told 60 Minutes he writes his report and answers questions as honestly as possible.
"I've always prided myself on being totally independent and I've never ever accepted a preferential supplier status from ACC," he said.
Dr du Plessis said only about one in 10 patients he see he believes should not be on ACC, and another two or three should be getting less than they are getting.
"If you had a bad injury, it doesn't mean you can't do nothing. You can always go back [to work] and do something. It's also part of rehabilitation to do things."
Acting director of clinical services Dr Peter Jansen told 60 Minutes there was "no list of preferred assessors".
"We use expert assessors because they write good reports that enable us to understand the situation that the client [is in]," he said.
"All doctors in New Zealand must adhere to same ethical standards and same professional standards, including providing assessments and advice that is not tainted by anything other than the clinical position in front of them."
Dr Jansen argued that sometimes assessors are not available - even in large centres such as Auckland - so experts need to be flown in.
"We don't have expert assessor in all parts of the country for ... medical, clinical, occupational therapy, housing, other types of assessments."
Dr Jansen said people are entitled to seek a review if they feel dissatisfied with a decision by ACC.
So far this year ACC has lost nearly 50 percent of cases that have gone to independent review, 60 Minutes reported.
Green Party ACC spokesperson Kevin Hague said the ACC board needs immediately change the way it commissions specialist medical assessors.
"There is no doubt that ACC has a standard practice of using specialist medical assessors who are likely to make an assessment favourable to ACC," Mr Hague said.
"The many claimants' stories I have on file show, in particular, it is common for ACC medical assessors to have views which are unusual in their specialty, and who are willing to offer opinions outside of their recognised scope of practice."
