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.

Transport networks, health systems and the Australian Tax Office were all among those affected as the outage dragged on.

About 1pm on Wednesday, the Optus Help account on X, formerly known as Twitter, confirmed some of its services were gradually being restored.

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

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

Tweet from @optus_help

However, the country’s second-largest telco also warned that some mobile phone users were having issues making emergency calls.

“If Optus customers need to call emergency services, we suggest finding a family member or neighbour with an alternative device,” it said.

Earlier, Optus had confirmed its landlines could not be used for triple-zero calls.

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.

“This is a very, very rare occurrence and when I have more information about the root cause, I will be the first to let all our customers know,” she told ABC Radio Sydney.

Bayer Rosmarin used WhatsApp to contact the station to give her first public statement earlier on Wednesday.

As services began to return, she contacted another Sydney radio station to say the cause of the outage remained a mystery.

“There is no indication that it has anything to do with a hack or cyber attack,” Bayer Rosmarin told 2GB.

“We’re still investigating the root cause. As you know, we work very hard 24/7 to keep our customers connected.

“This is a very unusual occurrence and as soon as we understand exactly what happened we will be forthcoming with details.

“We know how much everybody relies on our connectivity. We’re very, very sorry for this outage. we’re doing everything we can to bring it back up as quickly as we can.”

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 said.

“It is important at this time that people have their services restored as soon as possible.”

Rowland said customers might desert Optus, following the second major event within just months. She said consumers would make “judgments about the quality of service that they receive in a competitive market”.

That was backed up by Finder tech expert Angus Kidman.

“This could be a case of ‘two strikes and you’re out’ for Optus – customer loyalty is already on shaky ground following the data breach in 2022,” he said.

Rowland encouraged consumers, especially small businesses, to keep receipts in case they wanted to pursue any compensation.

The Telecommunications Industry Ombudsman also advised affected customers to lodge complaints if they had contacted Optus and were 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,” it said.

Unions have also lashed the telecommunications giant.

“Australians deserve better than to have one of the major national carriers leave them in the dark for hours, with no answers,” Communication Workers Union national assistant sSecretary James Perkins said.

“You cannot slash over 600 jobs in 12 months and not expect that to have an impact on services”

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 said 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.”

Tweet from @ato_gov_au

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 continued throughout the morning peak as services were restored.

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

The Royal Melbourne Hospital called a code yellow at all campuses, following Wednesday’s outage. It signals an internal emergency affecting services.

Virgin Australia told Travel Weekly its operations had also been hit.

“The Optus network outage has impacted approximately 40 flights, with delays of an average of 20 minutes. There have been no cancellations yet. However this may change,” the spokesperson said.

“We are proactively communicating with our guests via SMS, email, social media and our website.”

Virgin’s contact centre and other telephone services were also affected.

Numerous other businesses and organisations reported similar issues with phone lines and taking over-the-phone and other digital payments.

-with AAP