Prison authorities in two states and the Northern Territory lost track of criminals when recent Telstra outages caused their electronic monitoring devices to fail, the ABC has learned.

Key points: Tracking bracelets for 750 criminals went offline after two Telstra outages

Tracking bracelets for 750 criminals went offline after two Telstra outages It took four days to fix the problem in the first instance

It took four days to fix the problem in the first instance High-risk sex offenders were among those affected

The mobile blackouts of May 4 and May 21 resulted in the electronic ankle bracelets tracking criminals' movements to fail in South Australia, Queensland and the NT.

South Australia Correctional Services yesterday lost connectivity with the devices worn by 750 individuals for nearly two hours.

But it was the Telstra outage on May 4 which created a more prolonged predicament for authorities in the Northern Territory, where it took up to four days to fix the problem.

The electronic monitoring system is supposed to provide 24/7 monitoring to ensure criminals are obeying curfew orders and not entering exclusion zones.

The NT Correctional Services Department has confirmed that the outage affected devices used to monitor 85 individuals, including prisoners serving sentences at work camps, and people on community correction orders.

"A significant number of NT Correctional Services electronic monitoring devices were affected by an intermittent outage issue in the Telstra Mobile G4S network communications service that began at 1:10am on Friday 4 May 2018," a spokeswoman from the NT Correctional Services said.

"At 8:10am Friday, 4 May 2018, there were approximately 85 electronic monitoring devices identified as having unresolved communication issues and by 11:30am, 23 had been resolved. By Saturday 5 May, only 7 remained unresolved," the spokeswoman said.

"The safety of the public is always a priority and electronic monitoring bracelets are only one of a number of ways people on parole or community corrections are monitored.

"The issue of preventing or fixing Australia-wide G4S outages is the responsibility of Telstra."

A Telstra spokesperson said tracking devices "should automatically reconnect when the network is restored".

"In the case of all outages Telstra works to restore services as quickly as possible," the spokesperson added.

Unlike the Northern Territory, South Australia does not use the devices to geolocate prisoners, only individuals on parole, on bail or completing their sentences in the community.

Security contractor G4S, that leases the systems to government agencies, declined a request for comment.

Queensland monitoring devices affected

Electronic monitoring devices in Queensland were also hit by an outage on May 4, Queensland Corrective Services (QCS) has confirmed.

"QCS immediately put contingency processes into place to ensure public safety, and escalated the incident to the service provider for priority resolution," a QCS spokeswoman told the ABC.

"Offenders have no way of knowing when the devices are not operational.

The QCS confirmed that 299 devices worn by offenders, including high-risk sex offenders, were affected.

The spokeswoman said the organisation used other contingency plans.

Some of the affected prisoners reside on the periphery of the Townsville Correctional Centre. ( ABC News: Kathleen Calderwood )

"Offenders subject to GPS monitoring may be confined to their residence if there is a sustained loss of data connectivity," she said.

"The most serious offenders are monitored by staff on the ground at the Wacol and Townsville precincts.

"Checks on other offenders are made by physical and telephone checks in the event of prolonged outages."

The ABC understands that the May 4 outage affected QCS monitoring devices for 45 minutes and no serious breaches occurred.

The ABC has approached other states' corrective services agencies to clarify what impact the Telstra outage had on their respective monitoring systems.

Corrective Services NSW said they experienced no issues with their electronic monitoring system as a result of the Telstra outages on May 4 and Monday.

"Our system roams across the three major telecommunications networks so that, in the event one experiences an outage, the system will divert to the next," a spokeswoman explained.

Software fault caused outage, Telstra says

The impact on prisoner monitoring devices emerged as Telstra started to shed light on what caused Monday's massive network disruption to its 4G mobile phone services.

"We have identified that the initial cause of the disruption was a software fault which triggered multiple elements across the network to fail," a Telstra spokesman said.

"The network is designed to switch onto standby hardware, which it did. Following the failover however, a further fault caused an interruption which impacted 4G connections.

"There is redundancy built into these systems but this did not operate as intended."

The outage resulted in the failure of many National Bank and Commonwealth Bank EFTPOS facilities across the nation, the cancellation of several regional train services, and widespread problems with customers' mobile phone and data services.

Telstra said it was still investigating the root cause of the software fault.