From 2013 to 2016, Westpac contacted customers by mail and phone offering to help them shift money from other pension funds to its own, boosting its own holdings by about A$650 million ($440 million), the Federal Court said.
By "closing" sales and getting customers to transfer pension money in the same phone call, "there was an implied recommendation ... that the customer should accept the service", the three judges wrote in a ruling published on Monday.
The court gave ASIC and Westpac two weeks to agree on what public declarations the bank must give. If they don't agree, the matter will return to court to determine penalties for the bank.
The decision is a boost for the Australian Securities and Investments Commission (ASIC) which along with other regulators has struggled to land a major court win against banks.
A year-long public inquiry that ended in February gave a scathing assessment of the culture at the country's financial giants, also criticising regulators which it said had allowed the sector to reward overly aggressive, and sometimes deceptive, sales methods.
The Federal Court appeal tribunal dismissed a December ruling that Westpac's advice was only "general" and therefore allowed - in what was then an embarrassing loss for ASIC given it took place while the inquiry was still running.
ASIC said the ruling "provides clarity and certainty concerning the difference between general and personal advice for consumers and financial services providers".
Westpac said it was carefully considering the judgment.
ASIC lost against Westpac again in August when a court found the regulator had failed to prove the bank approved hundreds of thousands of mortgages without adequate credit checks.
Last month, the Australian Prudential Regulation Authority, the country's banking regulator, lost a landmark case against IOOF Holdings Ltd in which it accused the wealth manager's executives of breaking the law by using members' money to top up investment losses. The Federal Court found the action was lawful.
($1 = 1.4676 Australian dollars)
(Reporting by Byron Kaye; Editing by Edwina Gibbs)
By Byron Kaye