Every WA energy retailer found to have overcharged customers after Synergy's $40m bungle

Every energy retailer in Western Australia has overcharged customers by taking automatic payments from closed accounts, an investigation has found.

It was revealed in October that state-owned utility Synergy had overcharged 174,000 customers a total of about $40 million since 2009 , affecting mostly residential customers.

An investigation by the Economic Regulation Authority (ERA) has now determined all six other energy retailers in WA have overcharged thousands of customers in just the past two years.

While the watchdog's investigation focused primarily on Synergy, WA's largest energy provider, it also requested data from other electricity and gas providers.

The ERA collected data over the past two years, from when industry codes were implemented for electricity suppliers in Februar