Millions of Australians were left without mobile and internet after a network failure at telecoms firm Optus.
The failure caused transport delays, cut hospital phone lines, and shut down payment systems.
Optus, the country's second-largest provider, said more than 10 million people and thousands of businesses were affected.
Services were restored after around 12 hours of disruption. Optus said there was no evidence of a cyber attack.
The firm blamed a "technical network fault" and said more time would be needed to investigate the root cause.
Wednesday's disruption was reported around 04:00 (17:00 GMT). It took until around 18:00 for services to come back online.
Kelly Bayer Rosmarin, the company's chief executive, said it had not yet identified what went wrong.
People across Australia were unable to call emergency services and critical helpline numbers. Train services in the state of Victoria were also temporarily crippled.
The failures also affected other providers that use the Optus network, including Amaysim, Aussie Broadband, Moose Mobile, and more.
One Optus customer told the Australian Broadcasting Corporation the incident had left her unable to receive important updates about her father's cancer treatment.
"I'm just waiting for results, and I can't even get those through," Danielle Hopwood said.
Another customer, Annie, told local radio she found out about the disruption when her cat's automatic wi-fi-enabled feeder failed.
Ms Rosmarin apologised for the network failure, telling ABC News: "Until we've done a full, thorough, root-cause analysis, we really can't provide more information.
"What I can say is that it was a technical network issue, and that our teams have worked very, very hard to get services restored as quickly as they possibly could."
She also rejected claims from unions that 600 job cuts were partly to blame.
"I don't think that that's at all related," she said. "We're very sorry that this occurred and we will take all the learnings."
The company last year suffered what was believed to be the biggest data breach in Australian history, as a result of a cyber attack.
