Optus mobile and internet services are gradually being restored after a nationwide outage left millions of Australians disconnected for close to nine hours. 

The network dropped out about 4am on Wednesday, with Optus customers and businesses 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.

“This may take a few hours for all services to recover and different services may restore at different sites over that time,” the company statement said.

“We reiterate our apology to customers for the nationwide service outage that has occurred this morning.”

Close to 10 million Optus customers had their personal information stolen when the company’s data system was breached last year. 

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.

“We’re really sorry that this outage has occurred and we’re working to restore services for our customers as a priority.”

Ms Bayer Rosmarin confirmed people could not 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 any compensation.

The Telecommunications Industry Ombudsman also advised affected customers to lodge a complaint if they have contacted Optus and are unhappy with the response.

“We can help you with refunds for the time you have been unable to use your service, compensation claims and disputes about your contract,” the statement read.

Greens Senator Sarah Hanson-Young has 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 a big corporation – millions of customers, billions of dollars within the economy at stake, lives at risk,” she told reporters on Wednesday.

“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.”

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

All metropolitan services were stopped from about 4.30am because of a communications fault across the train network.

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.

“We thank passengers for their patience while trains return to their normal timetable.”

Service NSW call centres, Victoria’s virtual emergency department and Northern Health hospital phone lines were also down during the Optus outage.