The company also admitted misleading customers about how promotions worked and when they expired, according to a Federal Court judgment.
Advertisements placed between July 2019 and August 2023 that promoted store credit, or "StoreCash", likely resulted in "millions" of breaches of consumer law, Justice Michael O'Bryan said.
"The Good Guys sent over 5.3 million text messages and over 6.4 million emails to consumers advertising the Store Credit promotions," Justice O'Bryan said.
Customers were offered credit if they purchased a specific product or spent a minimum amount.
The Good Guys admitted it had failed to warn customers they were only eligible for credit if they opted in to marketing messages from the company and the credit expired in a short time, usually seven days.
The misleading promotions might have prompted people to make purchases when otherwise they could have chosen a competitor, the consumer watchdog said on Tuesday.
"When advertising promotional offers, all businesses must clearly disclose any key terms and conditions or limitations to avoid misleading consumers," Australian Competition and Consumer Commission chair Gina Cass-Gottlieb said.
As well as paying the $13.5 million penalty, The Good Guys has agreed to give extra store credit to customers who were not properly told about when the offer expired.
The compensation is of a higher value than what the customers were initially entitled to and has an expiration period of more than a year.
In a statement to investors after the judgment, The Good Guys parent company JB Hi-Fi said it had co-operated with the ACCC and took its legal obligations seriously.
"The Good Guys has always sought to provide value and benefits to its customers and has always prided itself on its high levels of trust with consumers," the company said.
Several of the breaches were self-reported to the consumer watchdog once The Good Guys became aware its promotions were under scrutiny, the court confirmed.
"There is no evidence that The Good Guys intended to breach or flout the law by the contravening conduct," Justice O'Bryan said.
The judgment comes as one of the company's major competitors faces its own legal woes for allegedly misleading conduct.
Harvey Norman is the subject of two class actions, including one announced on Tuesday over allegedly false interest-free loans.
Carter Capner Law alleges hundreds of customers were "slugged with hefty fees and charges and supplied with credit cards they hadn't requested".
Another class action, filed by Echo Law in September 2024, alleges Harvey Norman "sold unnecessary and worthless extended warranties" to tens of thousands of customers.
The company is fighting the action.
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