The outage, which began around 4am on November 8, could take hours to restore entirely, Optus said.

"Different services may restore at different sites over that time," an Optus spokesperson said.

"No action is required and services will be automatically restored."

Triple Zero emergency calls were not working from Optus landline telephones but mobile phone calls to 000 connected if another carrier was available in the area.

"If Optus customers need to call emergency services, we suggest trying to find an alternative device," the spokesperson said.

The telecommunications provider does not know the root cause of the outage but federal communications minister Michelle Rowland said the fault was likely "deep in the core" and "fundamental to the network".

Services relying on Optus were also suffering network issues including Aussie Broadband, Dodo and Amaysim.

Ms Rowland said her office was in communication with Optus and encouraged the telecommunications company to be "transparent and timely in the updates it's giving to its customers".

"Consumers are hungry for information," she said.

"They really need to know how to get on with their day and plan their lives."

Optus CEO Kelly Bayer Rosmarin reportedly told Ms Rowland that "a number of problems have been identified" and her team was working on a solution.

Ms Rowland was unable to confirm if a cyber-attack was responsible for the outages.

Monash University Dr Bill Corcoran said information from Optus suggests the issue was with networking and control of the infrastructure.

"What's interesting about this outage is that the fundamental links - the optical fibres, the cell towers, etc - all seem to be fine and ready to run," he said.

"What this says is that the really expensive stuff to put in, the physical cable infrastructure - is good."

Ms Rosmarin called ABC Sydney via WhatsApp due to the service issues.

She said she has "no more information" on the cause of connection problems and they've tried "a number of paths of restoration" that haven't worked.

On its website early on November 8, Optus said its engineers were investigating a network fault impacting customers.

"We understand connectivity is important and apologise for any inconvenience caused," an Optus statement posted to X said.

The Telecommunications Industry Ombudsman (TIO) said it could forward complaints to Optus if customers were unable to contact the telco.

Customers who have contacted Optus and were unhappy with the response are able to make a complaint through the ombudsmen.

"We can help you with refunds for the time you have been unable to use your service, compensation claims and disputes about your contract," TIO said.

Disconnection reports peaked just before 6am with almost 9,000 people lodging their service failures with Downdetector.

Complaints from customers across the country poured in with many saying they are unable to start their workday due to connection issues.

Ms Rowland said a "number of government agencies are being monitored" for issues relating to outages including state and federal departments.

Access to crucial health services have been affected including Service NSW call centres.

South Australian premier Peter Malinauskas said "it is a substantial inconvenience" to patients but not one that has affected core health services.

South Australians struggling to access mental health triage services were the state's greatest concern and patients experiencing a mental health emergency should access Triple Zero from a mobile phone, he said.

Alert SA users are encouraged to find an alternative source of information regarding bushfire risks in the state.

Phone lines across Northern Health's hospitals and virtual emergency department in Melbourne's north were also down.

The outage is affecting a range of ACT government services, including Access Canberra contact centres and health service phone lines.

ACT buses are running as normal however, real-time information through the NXT bus timetable is not updating.

Melbourne train services were also reportedly interrupted with all metropolitan lines stopped from 4.30am due to a communications outage across the train network.

Metro Train services started to resume just before 6am but major delays were continuing during the morning peak as services were restored.

"There will be significant delays to services and select delays are likely as trains get back into position for the normal schedule," a transport department statement said.

"We ask passengers to check the PTV app or website for information about service changes and cancellations."

Commonwealth Bank call centres were reportedly unable to receive calls but a number of their other services remain online.

"Our ATMs, NetBank, CommBank app, branches, CommBiz and merchant terminals are still available," Commonwealth Bank said.

ANZ and Westpac banks said they were both aware that some Optus customers may struggle to reach call centres but online banking, ATMs and branches were operational.

IN OTHER NEWS:

Police from Castle Hill in NSW have warned drivers to be vigilant and patient on the roads with many motorists losing access to Google Maps, Waze and Apple Maps.

"Some of the roads may not exist on these things if you still have them," police said.

Traffic jams and congestion could be due to Optus outages as one man found while waiting in line for parking.

More to come

With Australian Associated Press