Fairfax Media reported that the incident had raised questions about Telstra's reaction to the situation as it meant some who called the number had to wait as much as nine hours to be called back.
During the incident, operators who answered triple-zero calls directed them to a recorded service to confirm if they were genuine.
Vocus said in a statement to iTWire: "Analysis identified that one of our customers PABX systems was compromised, enabling an external party to attempt international toll fraud.
But the company said many of the calls included a “000” prefix which were routed to emergency services. It said that steps had been taken to prevent another such occurrence.
"Toll fraud is a significant problem and can be complex and time consuming to resolve. Vocus takes fraud awareness very seriously and works closely with customers to assist them to keep their business safe from toll fraud attacks," Vocus said.
This is the second incident to affect triple-zero services in May. A damaged cable caused an outage on 4 May, and the Australian Communications and Media Authority is conducting an inquiry into the incident.
Telstra has to comply with the Telecommunications (Emergency Call Service) Determination 2009 and the Emergency Call Service Requirements Industry Code C526:2011, according to the ACMA.
A breach of the Determination could result in a fine of $250,000 per breach. Future compliance could be sought through a court-enforceable undertaking.
The Fairfax report said Telstra was required to inform other telecommunications operators about events like this under the Emergency Call Service Code.
It said other providers were only informed of the 26 May incident after 5.30pm. Telstra has been contacted for comment.
An Optus spokesperson told iTWire that Telstra had informed it on Saturday evening of a triple-zero issue which may have affected Optus customers.
iTWire has also contacted Vodafone to inquire when it was notified about the incident.