Westpac has increased the amount refunded to mortgage customers left out of pocket by a glitch that meant they were not paying down their loans.

Westpac in December 2017 said it was setting aside $11 million for remediation after 13,000 customers did not have interest-only mortgages switched to principal and interest as scheduled, with the result that they were not paying down their debt.

Westpac told AAP on Tuesday it had increased provisioning for the issue.

“We apologise to customers impacted and want to assure our customers that we have since introduced an automated switching process to prevent this error occurring again,” Westpac said in a statement.

It did not confirm whether the number of customers affected had changed, but letters sent to customers in recent weeks put it as high as 40,000 – including at subsidiaries BankSA, St George, and Bank of Melbourne.

The Herald Sun on Tuesday reported that 70 per cent of customers have already been remediated.

The newspaper said 30,000 customers were ahead of their repayments and suffered no financial impact.

Westpac has not revealed the updated cost of the mortgage glitch, but in May’s first-half report said it was included in the $617 million set aside in the period as part of wider remediation action following the financial services royal commission.

Westpac had already booked $281 million in remediation costs in its full-year results in November and indicated at the time the cost of the mortgage glitch could exceed initial expectations.

Westpac shares slipped 0.5 per cent to $27.88 by 1110 AEST, but have still gained more than 14 per cent so far in 2019.