Exetel fined $694K for failing to comply with laws that protect customers from scammers.
Scammers were able to gain control of mobile number services and access consumers' bank accounts, with reported losses of at least $412,000.
The financial penalty is the largest to date for contraventions of these rules.
An Australian Communications and Media Authority (ACMA) investigation found the company breached the rules on 73 occasions in June and July 2024 after scammers were able to manipulate its systems to bypass parts of the required identity verification processes.
"This was a sophisticated attempt by bad actors using credentials obtained illegitimately from external sources unrelated to Exetel or its customers," said an Exetel spokeswoman.
"Whilst there was a gap in our mobile number porting process for a short period, we worked quickly to identify and correct that gap by July 2024.
"We apologise for any inconvenience that the impacted consumers may have experienced.

Should have fined more