Millions of customers of Australia's second-largest telecommunications company, Optus, were affected by an outage of mobile phone and internet services on Wednesday morning.
The outage also impacted hundreds of thousands of businesses, as well as some government services, public health systems and public transport systems.
As services are being gradually restored by the company, let's take a look at what we know about how the outage unfolded.
When did the outage start?
Many Optus customers woke up to see their phones had lost connectivity on Wednesday morning, and the outage was first noticed by Optus at 4:05am AEDT, according to the company's CEO.
Customers managed to vent their frustrations on social media using other means, before Optus confirmed the outage.
"Optus is aware of an issue that may be impacting some of our mobile and internet customers," the company said in a statement.
On social media platform X, the Optus support team said it was "working urgently to restore services" and apologised to customers.
Who was affected?
About 10 million Optus customers are believed to have been impacted, as well as about 400,000 businesses and some government, health and transport systems.
Many businesses were unable to allow customers to make EFTPOS payments.
Customers of smaller mobile service providers that use parts of the Optus network were also impacted.
Some Australian banks had issues receiving customer calls, and some people reported issues with two-factor authentication services used by banks and online retailers to confirm their identities and transactions.
Monash University's IP Observatory said the outage was most severe in Victoria and Queensland.
The Australian Department of Home Affairs said the outage had impacted "phone lines, mobiles and some web services for the Department of Home Affairs and Australian Border Force".
Communication systems in some state government services, hospitals and local health systems were also impacted.
All major hospitals in Melbourne were hit, including Victoria's Virtual Emergency Department. Northern Health, which manages hospitals in the city's outer north, reported phone lines into its campuses were down.
All of Melbourne's train services stopped temporarily due to a communications outage, and Metro Trains said it was unable to rely on its back-up system, which used the Optus mobile network.
Train services later resumed, but passengers were warned there would be ongoing delays
South Australian Premier Peter Malinauskas said SA Health, SA Water, and Service SA had all been affected by the outage.
How long was the outage?
Monash University's IP Observatory said its data showed the outage lasted for about seven hours.
At 12:55pm AEDT, as some Optus customers noticed their mobile reception returning, the company said some of its services were being "gradually restored".
"This may take a few hours for all services to recover, and different services may restore at different sites over that time," Optus said.
"We reiterate our apology to customers for the nationwide service outage that has occurred this morning. We will continue to provide updates as we have information available."
Communications Minister Michelle Rowland said the government was seeking further information from Optus, and urged it to communicate with its customers.
Shadow Minister for Communications David Coleman called the outage "a catastrophic incident".
"There's no precedent for an incident of this kind," he said.
Could Optus customers call triple-0 during the outage?
Emergency calls worked on most, but not all, of Optus customers' mobile phones while they didn't have reception, but there were widespread issues with making triple-0 calls from Optus landlines.
In a statement during the outage, Optus said:
"We encourage any customers who need to contact emergency services to use a mobile line to call 000. Optus can confirm that triple zero (000) calls will not work from an Optus landline (fixed line telephone). Mobile calls to 000 will work if another carrier is available."
After services started returning, Optus said it was aware that some of its mobile customers were having issues contacting triple-0.
"If Optus customers need to call emergency services, we suggest finding a family member or neighbour with an alternative device," the company said.
Andrew Williams, the CEO of the Australian Communications Consumer Action Network, earlier told ABC News he had been assured by Optus that customers could ring triple-0 on their mobile phones, as the call would be handled by one of the other phone networks.
A carer told ABC Radio Melbourne that they had difficulty calling an ambulance on Wednesday morning.
"One of my patients had a cardiac arrest, I couldn't call an ambulance," he said. "I had to run out on the street and borrow a phone from someone walking his dog."
What was the cause of the outage?
The cause of the outage isn't clear yet.
Optus said it was working to identify the cause, and federal authorities said they did not believe it was the result of a cyber attack.
Optus CEO Kelly Bayer Rosmarin told ABC Radio that there was "no indication" of a cyber incident, and the company was working on "a number of hypotheses".
Communications Minister Michelle Rowland said she understood the fault was "deep in the core" of the Optus network.
"The core network basically encompasses everything from routing to electronics," she said.
"So it is a fault that is quite fundamental to the network. My understanding, having recently spoken to the CEO, is that a number of problems have been identified and Optus continues to work on this."
Mr Williams said a variety of things could cause outages.
"One of the major carriers had one a few years ago, where a fibre optic cable got cut. Others have been software glitches and software problems which crashed networks," he said.
Optus suffered a major cyber attack in September 2022, which led to more than 2 million customers having their personal identification documents compromised by hackers.
Loading...Will there be compensation for Optus customers?
Optus is yet to confirm whether it will offer monetary or other compensation to its customers, but its CEO told Nine Radio that it was "looking at what we can do to say thank you to our customers for their patience".
While there's no regulatory obligation to provide compensation, Telecommunications Industry Ombudsman Cynthia Gebert has told ABC Radio that businesses should keep track of any losses caused by the Optus outage.
"Keep a record of what you think are losses you may be experiencing as a result of this outage because it will have a significant impact on cafes and small petrol stations and things like that, who won't be able to process payments, because very few people carry cash anymore," she said.
"So gather the information you need in order to be able to contact Optus in the coming days and let them know what the impact of the outage has been on you."
Ms Gebert also called on Optus to "look at what is the right thing to do in each individual customer circumstance".
Last year's Optus data breach led to a class action lawsuit being lodged on behalf of affected customers, which is ongoing.
Law firm Slater and Gordon said it was pursuing a "substantial" compensation sum for those customers.
Some Optus customers were reimbursed by the company for having to replace their personal identification documents following the 2022 data breach, and some were offered free extra data and other benefits.