Telstra says its mobile voice and data services "have returned to normal levels", and they are working to restore services to customers who were impacted by the disruption.
The nationwide disruption affected some 4G mobile voice and data services nationally, and caused congestion for 3G services, Telstra said on Monday.
NSW Police have warned the outage may affect those trying to call triple-0 or the police assistance line and urged Telstra customers to use a phone belonging to another carrier or a landline if in need of assistance until the issue is resolved.
"The advice we have received indicates Telstra customer mobile phone calls to Triple Zero (000) should automatically be connected via another carrier; however, if you are having problems use a landline," a police statement said.
The telecommunications giant apologised for the inconvenience and said it is working with many of its customers to ensure their services are restored.
"We'll closely monitor network stability and performance and continue supporting our customers to restore their services," Telstra said on Monday.
"We're again deeply sorry for any disruption as we know how customers rely on our mobile services."
The cause of the outage is yet to be determined.
A map produced by fault tracking site Aussie Outages shows the problem is affecting a number of hubs around the country, including in Sydney, Hobart, Brisbane, Melbourne, Perth and Adelaide.
Earlier in May, Telstra customers were unable to make or receive calls following an issue with the company's 4G network caused by technical changes made ahead of upgrades to mobile traffic control equipment in Melbourne.
A couple of days later, triple-0 voice calls were affected across three states after a Telstra cable in NSW running between Orange and Bowral was damaged by what appeared to have been a lightning strike in NSW.
The issue resulted in problems with calls to the police, ambulance and fire brigade numbers in NSW, Victoria, Western Australia, South of Australia and Tasmania.
Telstra shares were 1.6 per cent lower at $2.80 at 3.30pm AEST on Monday.
