Telstra has confirmed an outage to its mobile network across the country as the provider is being flooded with thousands of complaints on social media.

The outage, which was first reported about 10am Monday, is affecting 3G and 4G users.

"We're aware of the issue impacting our mobile network. Sorry for any inconvenience," Telstra tweeted on Monday.

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. — Telstra (@Telstra) May 21, 2018

A map on Aussie Outages shows all major cities are affected. At 11.07am the website had more than 15,000 reports of Telstra network problems - mostly relating to internet, mobile and reception.

The most common complaint is that mobile reception has been reduced to emergency calls only.

AAP

We’re working to fix an issue impacting some mobile services. Sorry for the inconvenience. — Telstra (@Telstra) May 21, 2018

@Telstra How's this for service! Even the outages page gives an error message... pic.twitter.com/URkEFLDvT3 — Lawrence (@lwrncwds) May 21, 2018

NSW Police have warned the outage may impact Triple Zero calls or calls to the Police Assistance Line 131 444.

"Until issue is resolved, police are advising those affected, where possible, to use a phone belonging to another carrier or a landline when attempting to call these numbers," NSW Police tweeted.

Telstra said calls to Triple-Zero will connect over other carriers' networks where they have mobile coverage.

"Have not been able to receive calls or call out, text or use internet for over an hour now. Lucky I have access to a work computer to let family and friends know thier phone may not work if they have an emergency. This is so frustrating and dangerous," User J Kate wrote on Aussie Outages.

"Just what we need for a Monday morning!" Damian H wrote.

Thanks @Telstra for ruining my day and making me work overtime. I’ll be sure to send you an invoice — Matt (@mmatters90) May 21, 2018

This @Telstra outage has forced me to use a landline... send help pic.twitter.com/3Q93Rcugzd — Sammy Augeard (@saugeard) May 21, 2018

Not the first problem this month

Earlier in May, Telstra apologised to customers unable to make or receive calls following an issue with the company's 4G network.

Telstra then said the issue was caused by technical changes made ahead of upgrades to mobile traffic control equipment in Melbourne.

A couple of days later, Triple-Zero 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 11.48 AEST on Monday.