Simply Energy, which makes up 12 per cent of sales in the Australian power market, has paid out $2.5 million in fines after the external agents allegedly impersonated customers consenting to the switches in phone calls.

Victoria's Essential Services Commission said the agents transferred 525 gas and electricity accounts at 264 properties without obtaining their consent from July 2019 to April 2020.

Commission chair Kate Symons said many customers did not realise until after the switch had been made, while others tried unsuccessfully to prevent it.

"Fourteen customers complained to the company after receiving unexpected information packs, but Simply Energy still went ahead with the transfers," she said in a statement on Monday.

"Some customers spent hours trying to stop the transfers or to be transferred back to their original retailer."

It is the most blatant case of fraudulent sales contractors the regulator has dealt with since it was set up in 2001, Ms Symons said.

"This behaviour from rogue sales contractors undermines customer trust in the market," she added.

"This is the strongest action we have taken against an energy company and shows just how seriously we view customers being transferred by fraudulent means."

Simply Energy has now been issued penalty notices three times for not obtaining explicit informed consent before transferring customers.

"That factored into our calculation of the appropriate number of penalties to be paid and our discussions with the company which have centred on gaining assurance this won't happen again," Ms Symons said.

A Simply Energy spokesman said it had ended residential door-to-door sales activities in March 2020.

"The two field sales agents involved in these matters led deliberate and sophisticated criminal frauds, circumventing our policies and controls, and are the subject of criminal proceedings," he said.

"We have apologised to all affected customers, reversed and refunded any payments for fraudulent account transfers."

Australian Associated Press