The Financial Markets Authority says it and the Commerce Commission have formally agreed with Westpac on the steps the bank must take to refund fees to around 93,000 customers being overcharged $7 million.

it says Westpac did not correctly discount fees on a number of different banking packages and self-reported the issue to the FMA and Commerce Commission in December 2017. It identified more impacted customers over the following 12 months.

"The bank has agreed to refund all fees and charges that have been incorrectly charged and will take all reasonable steps to reach impacted customers."

The FMA says it was notified of the issue because of potential breaches of the fair dealing provisions of the Financial Markets Conduct Act 2013. The Commerce Commission was also involved because of possible breaches relating to consumer credit contracts under the Fair Trading Act 1986.

Westpac accepted that it may have contravened one or both of the Acts.

"It is understood that Westpac has already completed a significant part of the work to reach and refund impacted customer," the FMA says.

"The process for remediation is contained in an Enforceable Undertaking that Westpac has agreed to give the FMA and the Commerce Commission. The undertaking confirms that Westpac will notify the FMA when it has completed this process."