An investigation into power provider Synergy has uncovered widespread overpayments received by every single energy company in Western Australia.

The state's Economic Regulation Authority (ERA) issued a second compliance enforcement notice to energy company Synergy on Tuesday after 174,522 customers were charged a combined $40.4 million despite having closed their accounts.

The average affected Synergy customer is owed about $200 per account.

A select group of customers are owed about $5,000.

In investigating the wrongdoing, the ERA also found all six electricity and gas suppliers in WA overcharged thousands of customers over the past two years.

Only Synergy received a notice, however, as the company did not have the adequate processes in place to quickly notify and redress customers.

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