Nationwide outage leaves Optus customers offline

Mobile phone users are unable to make or receive calls and mobile internet services are down.

On the social media site X, formerly known as Twitter, Optus said it was working to restore services as a priority.

"We understand connectivity is important and apologise for any inconvenience caused," the statement said.

Melbourne train services were also reportedly interrupted due to the Optus outage.

All metropolitan services were stopped from about 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."