B.C. Transportation Minister Todd Stone says he is "angry" after learning that an ICBC computer mistake led to billing errors totalling an estimated $110 million in optional insurance coverage.

The Crown corporation estimates it owes its customers $36 million and an additional $3 million in interest for overcharging them.

At the same time, it lost an estimated $71 million when the same computer error led to some customers underpaying their premiums.

B.C. Transportation Minister Todd Stone has ordered ICBC to repay $36 million to motorists who were overcharged for insurance due to a computer error. (CBC)

ICBC says the mistake occurred as part of its move to a new computer system that caused incorrect vehicle descriptions to be applied to some optional insurance coverage for at least the last six years.

Six years is as far back as its computer records can "accurately recalculate historical premiums," it said.

“I am as angry, as I expect British Columbians to be, over this operational error at ICBC which for the past six years has been overcharging some customers and undercharging other customers for optional insurance," Stone said in a written statement.

Customers were overcharged $36M over six years due to a computer error in optional insurance billing. (CBC)

"This is not acceptable, and British Columbians expect more from their public auto insurer."

ICBC says the error affects the optional insurance coverage of 40,000 drivers in each of six years for a total of 240,000 motorists over the six year period, resulting in an average yearly overcharge of $21.

ICBC says that's less than two per cent of its customer base is affected by the overcharges.

It said the same error caused another three per cent of its drivers to underpay an average $34 per year.

Stone said ICBC will not be able to claim the estimated $71 million it lost.

"I have directed ICBC not to go after those who have underpaid, as it is not fair to expect them to pay retroactively for ICBC’s error," he said. "I also expect ICBC to fix its insurance management system so this error cannot happen again."

ICBC will be audited

Stone said he is also ordering an outside audit of ICBC's numbers, as well as an examination of its handling of the issue.

The minister said the error must not result in higher premiums to customers.

ICBC says it will be able to tell those affected within 90 days whether they overpaid or underpaid, and will begin emailing out those explanations along with cheques in July.

It said the error will not result in higher premiums.

The Crown corporation said updates to vehicle descriptions will be made in the coming months. As well, it's implementing new technology that will use vehicle identification numbers (VINs) provided by manufacturers to automatically capture each vehicle's make and model.

The vast majority, or 95 per cent, of ICBC's three million customers are not affected.