"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 has been factored into its latest levy consultation and accounts 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 per cent increase is proposed. That spreads the funding impact over 10 years.
Riley said ACC will 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 per cent at the start of 2002, rising as high as 8.11 per cent in 2008 and has since dropped to 2.84 per cent.
The number of affected customers was relatively consistent in each of the years, Riley said.
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.