Telstra customers have been offered more free data after an "extremely complex" network and modem connection error caused the fourth major network outage this year.
Irate customers have lashed out at the telecom giant - which prides itself on having the best network in Australia - over the fault which affected customers around the country.
It surfaced early on Friday morning, preventing some NBN and ADSL users connecting to the internet.
"Following disruption to some NBN voice and data services and ADSL services today, all services were restored just before 7pm EST this evening," Telstra said in a statement at 2030 AEST.
"The issue is extremely complex, but in simple terms there was a fault with the device that manages the interaction between our network and all of the different types of customer modems.
"We understand this has been frustrating for affected customers and we will be providing them with some additional data."
News Corp Australia reported that a leaked internal Telstra memo showed the outage affected up to 350,000 NBN voice and data customers in NSW, South Australia, Queensland and Western Australia and 25,000 ADSL customers in South Australia and NSW.
The memo attributed the problem to a large number of reconnection attempts from customer modems.
A Telstra spokesman said the memo "reflected a point in time".
"I can confirm that was an internal memo from early this morning," the spokesman told AAP.
"It reflected a point in time view, noting that these things change rapidly by the minute."
Telstra apologised to customers affected by the outage, the fourth service failure from the telco since February.
"We apologise to customers and are doing everything possible to restore services as quickly as possible," the spokesman said.
Irate customers voiced their frustrations on social media.
"Another outage. Telstra, you are an absolute joke. I don't want free data, I want my money back," Bridie Allen said on Facebook.
Ruth Townsend said, "Dear @Telstra, how long will this outage last? How come your billing department never has an outage? Love Ruth."
"Another outage, Telstra? We've finally gotten our business phone line working after a week of being down, and now no Internet. Our business is losing money and yet you are happy to keep taking ours," said Catherine Rees.
