AGL will pay back $785,000 to customers it misled, following a Federal Court ruling last week.

The fine was ordered after the court found AGL misled or deceived close to 30,000 South Australian customers between January and July 2012.

AGL was found to have engaged customers by phone, and told them they would get discounts if they signed on to a new energy plan.

But when rates subsequently went up, the discounts reduced, leaving some customers paying even more than the standard rates.

However, the Australian Competition and Consumer Commission (ACCC) did find that AGL's deception was “inadvertent” and unintentional.

ACCC representatives said a fine of $700,000 was appropriate, due to AGL's prior record and related findings.

The watchdog argued that publication orders should be laid down as well, so that AGL would be made to inform customers and the community at large that it had been found guilty of deceptive and misleading behaviour.

AGL has agreed to credit the accounts of existing customers and refund to any former customers signed-up on the same premise.