ACC will refund $100 million plus interest to more than 300,000 businesses, including the self-employed, after discovering it had been overcharging some businesses since 2002.

ACC has provided a tool on its website that lets people check if they may be due a refund, which is available here: ACC refunds

The two "disappointing" errors that led to the overcharging were discovered "early last year" business customer head, Phil Riley said.

But ACC had waited until now to fess up because it needed to fix the underlying issues and put in place a system to pay refunds, he said.

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A total of $36m plus interest will be paid to about 106,000 people who became self-employed after 2002 and who worked more than 30 hours a week.

They were incorrectly charged ACC levies in their first year of business when regulations meant they shouldn't have been, Riley said.

Another $64m plus interest would be refunded to 200,000 businesses that paid provisional ACC invoices and which were wrongly not given automatic refunds after their circumstances changed, he said.

TOM PULLAR-STRECKER/STUFF The levying errors date back to 2002 but were only discovered last year after ACC began a huge system overhaul.

Riley said the refunds would average $340 for the "first year self-employed" and $415 for business owners that were wrongly charged provisional levies, excluding GST, plus interest at the 90-day bank bill rate.

Those averages reflected the fact that some business owners would have owned more than business that had been overcharged.

The biggest refund of provisional levies would be just over $154,000. That business was among three "outliers" that would be due six-figure refunds, but a third of the provisional-levy refunds would be less than $100.

The largest refund for the newly self-employed would be $4000.

Refunds would start this month and Riley hoped all would be paid by April.

"We very much regret the overpayments and apologise to anyone who made a payment that was not required," he said.

"Our focus is now on making this right as soon as we can, and we have already begun contacting affected customers."

Riley would not be drawn on how much interest ACC would have to pay, saying that had not been calculated.

But the timing and amounts involved appear to suggest it will run into the tens of millions.

The interest was expected to be less than $100m, he said.

TOM PULLAR-STRECKER/STUFF ACC head of business customer service delivery Phil Riley said it "very much regretted" the overcharging.

ACC discovered the overcharging when implementing changes associated with its $669m transformation project, which is still underway.

It told the Government of the issues in December, but the previous government had not been advised, he said.

ACC had already moved to plug the financial gap caused by the refunds, factoring it into new levy rates that it put out for consultation late last month.

Its policies required it to spread its effort to recoup the lost revenue over 10 years, Riley said.

In August, ACC separately admitted that it had overcharged hundreds of part-time self-employed by classifying them as full-time.

Riley said he expected most people would be understanding about the latest errors.

"I think people will be disappointed. I don't think it will undermine the purpose of ACC – people get that. I think they will trust us to do the right thing from here."

Riley said the refunds represented less than 0.4 per cent of the money ACC had raised from employers and workers since the overcharging began.