A nationwide outage struck Australia’s second-largest telecommunications company Optus on Wednesday, wreaking havoc at peak hour as millions were left without phone and internet services and some trains stopped in Melbourne.

Optus, owned by Singapore Telecommunications Ltd., said engineers were investigating a “network fault,” but didn’t provide details about the cause. The crash stretched from Perth in Western Australia and Darwin in the north, to east coast cities including Sydney and Brisbane, according to a network tracker on Optus’ website.

Communications Minister Michelle Rowland said the impacts of Wednesday’s outage are “concerning” and she urged the company to keep customers updated “by whatever means possible.”

The outage exposed the country’s modern-day reliance on phone companies for a range of services. Train commuters stranded in Melbourne found themselves unable to call for ride-share services like Uber, while home workers were stranded without a web connection. Westpac Banking Corp. said it was unable to take some calls because of the crash.

The network failure is another blow for Optus, which last year was hit by a major cyberattack. That security breach exposed the details of almost 10 million former and current customers, while more than 2 million of them had identity document numbers compromised.

“Our teams are working urgently to restore services,” Optus said in a statement Wednesday. “We will provide updates as soon as possible. We apologize sincerely to our customers.”