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An industry watchdog had found all of Western Australia's energy retailers have overcharged hundreds of customers in the past two years. WA Government-owned Synergy was the first – and the only one to be dealt a breach notice by the regulator for its $40 million in overpayments. But now all seven of WA's energy retailers have been found to have overcharged customers. READ MORE: 'Exploded me up into the air': Moment shark attacks foilboarder on camera There's been $213,000 of extra cash taken from closed Horizon Power accounts, $175,000 from Kleenheat customers and $155,000 from Alinta Energy customers. WA Premier Roger Cook labelled it "very serious" and said the government was "extremely angry" over the situation. "You can have confidence that we will make sure that when mistakes are identified that we will rectify the situation and insist on it," Cook said. The Economic Regulation Authority (ERA) investigation found more than $4 million in overpaid bills since February 2023. But refunds are not that simple. "When you have to go back after a period of time and find the customer, the customer may have moved house, they may have changed their phone number," ERA chair Steve Edwell told 9News. The ERA has stopped short of compliance action, confident the money will be repaid as operators take steps to improve processes. Except for Synergy, which was handed a breach notice. The watchdog has given the energy retailer until the end of April to try and pay back $40 million to affected customers, or face a $100,000 fine. This includes money already owed to more than 174,000 customers since 2009. "It is a small penalty in the grand scheme of things," Opposition Leader Basil Zempilas said. "What's most important here is that there's no repeat offences." If you think you've been overcharged, the advice is to contact your provider. Otherwise, they're compelled to call you. DOWNLOAD THE 9NEWS APP: Stay across all the latest in breaking news, sport, politics and the weather via our news app and get notifications sent straight to your smartphone. Available on the Apple App Store and Google Play.