The telecommunications regulator has fined Optus for failing to follow anti-scam regulations, leaving some customers thousands of dollars out of pocket.

The troubled telco, which is reeling from the fatal triple-zero outage, failed to close a systems weakness that enabled criminals to target the phone numbers of 44 people with Coles Mobile accounts, operated by Optus.

The scammers then used these phone numbers to infiltrate bank accounts and steal money, an Australian Communications and Media Authority (ACMA) investigation found.

At least four consumers' bank accounts were targeted, resulting in a total of $39,000 stolen.

Optus will pay a penalty of $826,320 for the security failures that happened between September and October last year.

ACMA member Samantha Yorke said the fine was

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