ACC is to refund about $100 million to businesses for overpayments of levies going back as far as 16 years.
The overpayments were discovered in March last year as the corporation set up a new system to manage levies and it found that it had not been complying with regulations written nearly two decades ago.
More than 300,000 businesses are affected. About 106,000 are self employed, who were levied in their first year of business and will get back on average $340.
The rest are businesses, which provisionally paid their ACC levies in advance, but were not required to because they had shut down or restructured but not told ACC. They are owed an estimated $64m, with an average refund of $415.
"We very much regret the overpayments, and apologise to anyone who made a payment that was not required," ACC spokesman Phil Riley said.
"Our focus is now on making this right as soon as we can, and we have already been contacting affected customers."
Mr Riley said anyone thinking they may qualify for a refund should go ACC's website to check.
He said it could take up to April next year to complete the refunds.
The cost could rise considerably because ACC will pay interest on money owed, although Mr Riley would not speculate how much extra that might be.
He said the new system has fixed the problems. The cost of the refunds is being recovered through workplace levies over the next 10 years.