ACC will refund more than 300,000 business customers $100 million plus accrued interest after overcharging them for 15 years.
Between 2002 and March 2017, when the error was discovered, the state-owned workplace insurer overcharged about 106,000 people in the first year of being self-employed by a total of $34 million. Over the same period, some 200,000 businesses were overcharged $64m on provisional invoices.
The refunds for self-employed people will average about $340 excluding GST and $415 for firms' provisional payments.
ACC uncovered the issue when preparing to introduce a new levy system and has started tracking down customers so it can repay them.
Phil Riley, head of business customer service delivery, said ACC regrets and apologises for the mistake and urged people to update their contact details with the insurer to help the process.