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

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

An Optus spokesman said the company was working to restore services as a priority.

“We are currently working to identify the cause and apologise for any inconvenience,” the statement read.

“In case of an emergency, customers can still call 000.”

Federal Communications Minister Michelle Rowlands said she was yet to receive an update from Optus but it appeared to be a “deep and significant” network problem.

“I can’t speculate at this time about whether or not this is something to do with some sort of cyber hack,” she said.

“All I do know is that it is impacting on Australians, that customers do want to know what is going on.

“I would urge Optus to get that information out through any channels that are possible and to do that as a priority.”

Melbourne train services were also interrupted early on Wednesday, reportedly due to the Optus outage.

All metropolitan services were stopped from about 4.30am due to a communications fault 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.”

Phone lines across Northern Health’s hospitals and virtual emergency department in Melbourne’s north were also down due to the Optus outage.

-AAP