Optus customers across Australia have been hit by a major network outage on Wednesday morning (AEDT), affecting mobile phones and internet.

Melbourne’s metro train network was also thrown into chaos as the telco meltdown affected a back-up rail communications system.

Problems were first reported about 4am (AEDT).

“Our engineers are currently investigating a network fault that is impacting Optus mobile and fixed customers. We apologise for any inconvenience caused,” a statement on the Optus website read.

The company said in a later statement it was “working to identify the cause” of the nationwide outage.

“Optus is aware of an issue that may be impacting some of our mobile and internet customers,” it told the Nine Network.

“We are currently working to identify the cause and apologise for any inconvenience. In case of an emergency, customers can still call 000.”

Mobile phones are unable to make and receive calls. Attempting to call someone brings up a ‘Not registered on network’ notice.

Mobile internet services have also been hit.

Tweet from @optus_help

The ABC reported millions of customers and businesses across Australian had been affected.

Tech writer Trevor Long said it was one of the biggest telco outages in memory, affecting Optus’s mobile and home broadband networks.

“Businesses that rely on Optus for payments and communication, they will be crippled this morning,” he told Nine’s Today show.

“This is without question the biggest telco outage we have ever seen because it spans both home Wi-Fi and broadband.”

Across the country, businesses were reporting the effects of the outage. Melbourne’s Northern Health network said it had been affected.

“All phone lines into Northern Health campuses are currently being affected by an Optus outage,” it wrote on Facebook on Wednesday.

“This includes phone lines into Northern Hospital Epping, Broadmeadows Hospital, Bundoora Centre, Craigieburn Centre, Kilmore District Hospital, and Victorian Virtual Emergency Department (VVED). We apologise for any inconvenience.”

Geelong-based Barwon Health said all of its phone lines were also down.

Melbourne’s Moonee Valley Racing Club said it had been affected, and urged anyone wanting to get in touch to use email. The Flemington Racecourse-based Victoria Racing Club said all of its lines were down.

The Royal Automobile Association of South Australia posted to X (formerly Twitter) that its roadside assistance hotline was congested due to the outage.

“If you’re after roadside assistance this morning, please call 8202 4600 and we’ll connect you directly to our team,” it said.

“You can also book via the app.”

Tweet from @KitWalker021475

Customers posted complaints on social media – from Cairns to Melbourne and Perth – as they tried to find out what was happening.

The outage appears to have happened early on Wednesday. It also seems to have hit smaller telcos that use the Optus network such as Amaysim and SpinTel.

In Melbourne, commuter trains were stopped due to a communications outage across the city’s rail network, with ongoing delays expected.

All metropolitan services were stopped from about 4.30am. Trains started to resume just before 6am but major delays continued throughout the morning peak.

“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.”

Tweet from @metrotrains

Communications Minister Michelle Rowland said the federal government had issued a “please explain” to Optus.

“Connectivity is essential for Australian consumers and businesses. The impacts of this outage are concerning and has left many Australians feeling anxious,” she said.

“The government has sought further information from Optus about this development and when they expect services will be restored.”

-with AAP