The problem, identified around 3:30pm, meant customers were having intermittent problems making or receiving calls.
The company said technicians worked "as a priority" to restore services and issued a statement at 6:30pm saying the problem had been fixed.
"We apologise to customers and appreciate their patience," a company spokesman said in a statement.
"We will continue to monitor these services closely."
On Twitter a customer claimed to have smashed his phone in anger about the outage.
The outage comes shortly after the Singapore owned network started piling on new broadband customers with a $90 dollar broadband package that included six months of free Netflix.
This resulted in their recently expanded network being stretched according to Optus sources.
The latest outage has affected the Companies mobile, Voice, Pre Paid and Post Paid operations.
Customers are already venting their anger.
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"Awesome service Optus!!! Can't make work calls. Will u pay for the lost time??" wrote Stef Lesser.
"Your network sucks, why won't my phone ring!!" wrote another customer, Shane Dickinson.
"We've identified a fault which means you might experience issues with voice at the moment," the telco said on its website. "Our team are on to it, and aim to fix this as soon as possible. We're sorry.
In Newcastle several businesses who rely on the network for communication with their service personnel and trades are angry that the carrier does not have a backup.
A senior executive of one Newcastle Company "We are down we cannot communicate with our staff. This is a Company who brags about their reliability Vs Telstra yet they have no back up for essential business who rely on their network to run their day to day operations. We cannot text or voice call".