The company blamed "an error regarding the interpretation of allowable charges".
The Herald Sun understands consumers were slugged excessive contract exit fees from as far back as April 2012 through to August last year.
The retailer has written to those who were unfairly stung when they quit APG within the first year of a contract.
It is trying to compensate $754,242 that was overcharged to 11,610 accounts - an average of almost $65 each.
The price gouging only emerged after an Essential Services Commission industry wide website sweep and early termination fee policy check.
Energy giant AGL, which acquired APG in October last year, apologised.
"We have been working to remedy the situation and affected customers will be refunded in full over the coming months," spokeswoman Nicole Rizgalla said.
About 6500 so far have either received credits or will be sent cheques late this month.
But it is unclear whether all dudded consumers will be tracked as they include people who have changed address.
The controversy follows APG's $1.1 million fine for dodgy doorknocker tactics.
Under state regulations businesses can charge exit fees of up to $20, plus GST and a "proportionate" return of paid incentives such as rebates and gift and movie vouchers.
The Herald Sun reported in October that an excessive fee investigation was taking place but only now has the company's identity and full scale of the problem been revealed.
ESC spokeswoman Amanda Clark said APG had given a voluntary undertaking to identify and reimburse customers.
The State Government has previously ruled out an exit fee ban. The industry argues scrapping the charges would shrink price discounts offered in contracts.
Opposition energy spokeswoman Lily D'Ambrosio said tougher enforcement of retailers was needed to avoid rip-offs.
The Government is reviewing a range of consumer protections.
heraldsun.com.au 12 Jan 2014