Millions of Australians have been left without mobile and internet service after telecommunications giant Optus was hit by an outage.

Optus is the country's second-largest provider, with more than 10 million individual customers and hundreds of thousands more businesses.

The outage has caused transport delays, cut hospital phone lines, and downed payment systems.

Its cause is unclear, but Optus says there is no evidence of a cyber-attack.

The company made global headlines last year after it suffered what was believed to be the biggest data breach in Australian history, as a result of a cyber-attack.

Wednesday's outage was first reported around 04:00 local time (17:00 GMT) and in an update seven hours later, CEO Kelly Bayer Rosmarin said her team had not yet identified what went wrong.

"The teams are trying many different angles and we will not rest until the service is back up for our customers," she said, calling in to local radio via WhatsApp.

The firm – which has faced criticism for not communicating with customers amidst the outage – said it had started restoring some of its services in a later statement.

The incident has left people across the country unable to call emergency services and critical helpline numbers, while also temporarily crippling train services in the state of Victoria.

It is also affecting other providers which use the Optus network, including Amaysim, Aussie Broadband, Moose Mobile, and more.

One Optus customer told the Australian Broadcasting Corporation the incident has 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 outage through her cat, who was left breakfast-less when her automatic Wi-Fi feeder failed due to the network issues.

Australia's Communications Minister, Michelle Rowland, said it has been an anxious morning for many Australians and called for the company to be transparent and "timely" in their updates to customers.

"My understanding is that this is a fault deep in the core… so it is a fault that is quite fundamental to the network," she said.

Ms Bayer Rosmarin has apologised for the outage and said the company will provide updates throughout the day.