Telstra, the country's biggest telecoms provider, said service was being restored to a number of large customers but did not give details about the nature of the problem.
The telco said it's now investigating the issue, which is impacting "a number of enterprise customers nationally".
"Most customer services have now been restored, and we're working with our customers on the remaining few", a Telstra spokesman said.
Australia Post, St George Bank, NAB, Westpac and ANZ were also impacted. If you or your business are experiencing issues, tell us about it in the comments.
Both ATMs and EFTPOS terminals are believed to be affected. "We apologise for the inconvenience and thank our customers for their patience", a Woolworths spokesperson said.
"Due to the current Telstra outage we are experiencing challenges with EFTPOS payments in stores nationwide and are unable to receive calls to our contact centre", a tweet from Caltex Australia said.
"We're aware some branches, ATMs and services in Sydney metro are non-operational due to a broader network failure in the region", St George Bank said in a tweet. There is no need to call us if you are experiencing problems with these services.More news: Antarctic Glaciers Are Unstable, Can Slide Off Into Sea
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"All EFTPOS transactions, bill payments and banking services are back up and running".
"We're sorry if this impacts you".
Australia Post has also confirmed that Eftpos and other banking services are now disrupted, blaming a "supplier issue". The cause of the problem is under urgent investigation.
"We are aware of an issue affecting some of our services", CBA said on Twitter.
Customers complained to CBA via social media that their EFTPOS machines were displaying error messages.
The outage is affecting national retailers such as Woolworths, which is unable to process EFTPOS payments at many of its stores. "We're working closely with Telstra to rectify the issue as quickly as possible".
The supermarket giant says a Telstra network issue means its stores are having to process payments manually.