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Accident Compensation Corp 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 $64 million on provisional invoices.
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 regretted and apologised for the mistake and urged people to update their contact details with the insurer to help the process.
"We expect it may take until April next year to complete the refund process," he said. "We will begin refunding customers this month."
ACC has taken a provision for the refunds in its 2018 annual accounts which are due to be tabled in Parliament next week.
Riley said the refund had been factored into its latest levy consultation and accounted for a 1 cent increase in the work account, where a reduction is proposed, and less than 1 cent for the earners account, where a 2.5 percent increase is proposed. That spreads the funding impact over 10 years.
Riley said ACC would pay the 90-day bank bill rate at the year the invoice was due with monthly compounding interest, but wouldn't speculate on how large that bill might be. The rate was 4.88 percent at the start of 2002, rising as high as 8.11 percent in 2008 and has since dropped to 2.84 percent.
The number of affected customers was relatively consistent in each of the years, Riley said.
The refunds for self-employed people will average about $340 excluding GST and $415 for firms' provisional payments.
ACC didn't inform the minister until December last year and Riley said the previous administration wasn't informed.
The Crown entity will seek feedback on whether to adopt payments in arrears from the self-employed, matching the way they pay income tax.