Up to 300,000 people a year could be missing out on ACC cover, treatment or support a new report has estimated.
That's more than four times ACC's own estimates and it's prompted the report's authors to call for an independent commissioner to oversee the system, make it more transparent and keep better data.
They also want to see a change in the way ACC determines how an injury was caused.
In a report funded by the Law Foundation, lead author Warren Forster said while ACC estimated it made 70,000 decisions declining cover or entitlements, it did not keep any specific data about it.
Using previously published information, the researchers said the number of people missing out was actually far higher, with from 200,000 to 300,000 people not getting cover, treatment and other entitlements.
Mr Forster said part of the problem was how ACC accepted or declined claims based on how it determined what caused an injury.
A lawyer specialising in ACC disputes, Ben Thompson, said ACC held the upper hand when the cause of an injury was disputed.
"They've got tremendous resources, not the least of which is their own medical specialists in-house.
"They can get medical reports on tap, that's not an issue for them, trying to contest that as an individual ... for a lot of people that's really insurmountable."
The chief executive of ACC, Scott Pickering, told Morning Report that the new report shows the organisation does need to improve services to its customers.
He said the scheme can be difficult to navigate for both staff and customers at times, but he said people receive the treatment they are entitled to which was outlined by the legislation the organisation operates within.