Customers should be able to access their services after an Optus spokesman confirmed services had been restored by Wednesday evening.
The network dropped out about 4am on Wednesday, with Optus customers and businesses unable to connect to the internet or make or receive calls.
Optus customers and businesses were unable to connect to the internet or make or receive calls.
An Optus spokesman confirmed some of its services were gradually being restored from about 1pm.
"Optus sincerely apologises to customers for today's outage," the spokesman said.
"We know that customers rely on our services, which is why the whole team at Optus has been working hard to fix this."
Close to 10 million Optus customers had their personal information stolen when the company's data system was breached in 2022.
There was no indication Wednesday's outage was the result of a cyber attack, chief executive Kelly Bayer Rosmarin said.
"We do everything we can to give great service to our customers," she told ABC Radio Sydney, calling into the station via WhatsApp.
"We're really sorry that this outage has occurred."
We're aware of an issue impacting Optus mobile and nbn services and are working to restore services as quickly as possible. We understand connectivity is important and apologise for any inconvenience caused.— Optus Help (@optus_help) November 7, 2023
Optus believes a network fault was behind the outage, but the root cause is still being investigated.
People were unable to make calls to triple zero on Optus landline devices during the outage, although it was still possible to do so on a mobile phone.
Federal Communications Minister Michelle Rowland said Optus needed to be transparent with its customers, who were experiencing "a high level of anxiety and frustration".
"Consumers will be making judgments about the quality of service that they receive in a competitive market," she told reporters.
"It is important at this time that people have their services restored as soon as possible."
Ms Rowland encouraged consumers, especially small businesses, to keep receipts in case they choose to pursue compensation.
The Telecommunications Industry Ombudsman also advised affected customers to lodge a complaint if they had contacted Optus and were unhappy with the response.
Greens Senator Sarah Hanson-Young called for a Senate inquiry into the outage, saying it was important for Australians to have confidence in essential phone and internet services.
"This is not a small matter and the parliament will have to look at what Optus can and should be doing ... and there needs to be consequences," she told reporters.
Michelle Rowland advised Optus customers to keep receipts if they want to pursue compensation.
South Australian Premier Peter Malinauskas said his government would reconsider its contracts with Optus because reliability was important.
"They will have to explain for themselves and I think they should be held to account," he told reporters.
National Retail Association director Rob Godwin said the outage was a blow to retailers and small businesses seemed to be the most adversely affected.
"EFTPOS is down for non-Optus users as well, and it can't be fixed today since some of the big banks are uncontactable," Mr Godwin said earlier on Wednesday.
"This is costing businesses thousands of dollars in sales."
All Melbourne metropolitan train services were stopped from about 4.30am on Wednesday because of a communications fault across the train network, reportedly due to the outage.
Metro Train services started to resume just before 6am but major delays continued throughout the morning peak as services were restored.
"We apologise to our passengers for the delay to their travel this morning," Metro Trains chief executive Raymond O'Flaherty told AAP in a statement.
Service NSW call centres, Victoria's virtual emergency department and Northern Health hospital phone lines were also down during the outage.
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