The outage affected EFTPOS terminals and access to ATMs across the country for "a number of enterprise customers nationally", a Telstra spokesman told AAP.
A Telstra issue is causing problems for Commonwealth Bank and Westpac customers nationwide.
"We're sorry for any issues caused and will provide an update when we know more", he said.
Meantime Comm Bank says its website and app should be working but has confirmed issues with ATMs, EFTPOS and some in-branch services.
Caltex said its EFTPOS payments were being affected; Woolworths said electronic payments were being disrupted; while Australia Post said EFTPOS payments over $100 and other banking services were on the fritz, while withdrawals under $100 and all deposits were still working.
"Some services are returning to normal and we thank customers for their patience".
Telstra has confirmed this afternoon that a machine-to-machine network is experiencing a nation-wide outage, which is affecting ATMs, EFTPOS terminals and more.
Westpac said some branches, ATMs and services in Sydney were non-operational due to a broader network failure in the region.
"All EFTPOS transactions, bill payments and banking services are back up and running". We'll keep you updated as we know more. The outage then cascaded into a higher number of calls than usual into its call centre.
The spokesperson also said the cause of the problem is not internal but due to "one of [its] telecommunication providers".
Commonwealth has released a statement on social media, informers customers that they are aware that their machines are flopping and they're doing their best to resolve the issue.
Retailers such as Woolworths are also reportedly being affected by the outage.
"Stores are continuing to trade as our teams are still able to process payments manually at this time".
"My understanding is, and this is unconfirmed from Telstra, the Telstra COIN Network, which is what they call the payments network, has had an issue today".