Updated May 21, 2018 15:14:38

Telstra says its mobile voice and data services have "returned to normal levels", after another outage crippled its 3G and 4G mobile phone networks across Australia this morning.

Network-monitoring website aussieoutages.com showed a large number of problems in Brisbane, Sydney, Melbourne, Adelaide and Perth, with other hotspots showing across Australia's population hubs.

There were also widespread complaints about Telstra's outage page not working, and NAB and the Commonwealth Bank both said they were aware the problem was impacting EFTPOS facilities.

Several train services in Victoria were cancelled due to communication difficulties, and roadside assistance services say they were also significantly impacted.

But the telco said triple-0 calls should have been unaffected, with calls connecting over other carriers' networks where available, and it apologised for the inconvenience.

Infographic: A map showing reports of outages across Australia's population hubs at 10:30am AEST. (Supplied: aussieoutages.com)

The company responded to customer complaints on Twitter, saying: "The issue is impacting some 4G mobile voice and data services nationally, which is also causing congestion for 3G services".

"We're working to resolve the issue and are investigating the root cause. We're sorry for the inconvenience."

At 12:16pm AEST Telstra said the situation was improving, and just after 2:00pm said "mobile voice and data services have returned to normal levels".

"We'll closely monitor network stability and performance and continue supporting our customers to restore their services," it added in a statement.

"We're again deeply sorry for any disruption as we know how customers rely on our mobile services."

Infographic: Telstra's outage page was also showing error messages. (ABC News)

One man, Dion Costigan, said on Twitter he was "Getting a little tired of this happening and has been for months".

The outage comes after a similar incident on May 1, when a nationwide issue prevented thousands of customers from making or receiving calls for several hours.

The Government is investigating Telstra's triple-0 outage on May 4, which saw emergency calls from around the country go unanswered.

Telstra's share price was down another 1.4 per cent to a fresh seven-year low of $2.81 shortly before midday (AEST) following the outage.

People were reporting on Monday morning that only the SOS services were available during the current issue..

Topics: information-and-communication, mobile-phones, australia

First posted May 21, 2018 10:50:49