Customers have been overcharged $40m over the course of 15 years by Western Australia’s state-owned electricity retailer.

Synergy revealed Friday it had been charging customers for non-existent bills since 2009.

The company says almost 174,000 people - mostly residential customers - have been overcharged, with a majority being owed less than $100, though 467 people are owed more than $5000.

“We have a responsibility and obligation to detect and return customer overpayments to accounts quickly,” Synergy chief executive Kurt Baker said.

“We will make this right for our customers, and we will take all possible steps to refund overpayments and rectify this situation.”

Synergy had “allowed customers to continue to pay to their closed account, even in the absence of a bill”, Mr Baker said.

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