A lawyer representing ACC claimants says the corporation has been acting illegally by contracting out its dispute resolution services to a private company.
Dunedin barrister Warren Forster said this calls into question thousands of cases where people have lost a dispute over their claim.
The Accident Compensation Corporation contracts the private company Fairway to resolve disputed claims.
However, Mr Forster claims Fairway has no legal authority to do this and that there is no independence from the corporation.
He said neither the Minister for ACC or the corporation's board had provided written authority for ACC to delegate its review powers to Fairway.
Mr Forster said by not issuing formal delegations and not having people legally authorised then the claimants would win the case.
"This could effect every review since the legal delegation which was in place was stopped," he said.
Everyone who had a review hearing, where the reviewer had not been legally appointed, was deemed to have the decision in their favour and won their review, he said.
"There's a large number of people who are entitled to a deemed review decision, because the person who took all the steps supposedly on their case had no legal authority to do so."
It is not clear how far back this problem goes or how the situation came to be Mr Forster said.
He said he had asked ACC and Fairway for evidence supporting the legality of their reviews which they have not given to him.
Mr Forster also questioned the independence of Fairway from ACC and said that there was a lack of transparency on how the two operate and that their relationship required greater scrutiny.
Mr Forster claimed there were requirements on the Fairway reviewers that certain numbers of cases get dealt with within a certain period of time and that ACC was controlling the performance of the reviewers.
"Reviewers are being told here's what ACC's policy is and we expect you to do what ACC says," he said.
He said if a reviewer went against ACC it started a cycle of reporting and their work was peer reviewed and that they were under pressure from the corporation.
"I've seen on individual claim files where the reviewer writes to ACC saying what do you want me to do with this?"
ACC said in correspondence with Mr Forster that ministerial approval was not required for it to contract review services to Fairway.
Mr Forster told Morning Report those who thought their case may have been dealt without lawful delegation to contact either Fairway or ACC themselves.
"If there is no evidence provided of that lawful appointment, then everyone is entitled to go through the process of consideration of whether they have a deemed review decision in their favour. So it is a big problem, there needs to be some leadership from ACC."
Minister of ACC Iain Lees-Galloway would not provide comment as he was still waiting to receive advice.