Consumers listed on the register may receive some telemarketing calls from companies that they have an existing relationship with, but they can withdraw consent at any time, the Australian Communications and Media Authority (ACMA) said.
At this point, it is up to Westpac to have systems in place to track updates and the status of consent.
ACMA's investigation found that although Westpac had an existing relationship with the consumers concerned, its procedures for recording the withdrawal of consent had failed.
Subsequent telemarketing calls were made to these consumers despite the withdrawal of their consent, according to ACMA chairman Chris Chapman.
"'Don't call me again' means just that," Chapman said.
"Telemarketers should have appropriate systems and procedures in place to ensure that calls are not made to customers who have previously indicated their wish not to receive these calls.'
ACMA said Westpac "has cooperated fully" with the regulator and has implemented a number of new procedures to address the issues identified in the investigation.
"ACMA is pleased to note that there have not been any further complaints about prohibited calls made by Westpac since these changes were put in place," Chapman said.
"[We] will be monitoring Westpac for ongoing compliance with the legislation".