Telstra says its mobile services are being "progressively restored" after the telco suffered its second widespread network outage in a month.
The nationwide disruption is impacting some 4G mobile voice and data services nationally, and causing 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 apologise for the inconvenience and are working to resolve the issue as soon as possible," Telstra said on Monday.
"We are still investigating the root cause.
"Our 3G and 4G voice and data traffic volume continues to be progressively restored nationally."
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 1.4 per cent lower at $2.81 at 1339 AEST on Monday.