The alleged Darwin gunman was wearing an electronic monitoring bracelet as he shot and killed four people, according to the Northern Territory Police Commissioner.

The alleged shooter was on parole and the NT Government has ordered a review of every prisoner who is on parole in the territory, including those fitted with a GPS tracking device.

So, how does the tracking work? And who is monitoring the parolees who wear one?

NT Corrections confirmed more than 100 people were on parole in the Territory, and of those, 46 were being "actively monitored".

In NT, those devices are managed by an outsourced international security company.

That company is G4S, which according to its website supplies, installs and conducts "monitoring of equipment and offenders in the community and prisoners at a work camp in Tennant Creek".

In a statement to the ABC, G4S said it had provided this service to the NT since 2014 and similar services to South Australia since 2000.

Are GPS monitoring devices reliable?

Yes. According to Dr Marietta Martinovic, an expert in electronic monitoring at RMIT University who wore a device for her research.

She said more than 90 per cent of people fitted with devices globally complied with the rules associated with it.

"The vast majority of people do the right thing," Dr Martinovic said.

"People who are the most serious offenders, they actually seem to comply with wearing the technology the best; that is, they don't seem to engage in further offending."

The monitors need to be charged for about an hour a day to work properly.

The devices may become ineffective in remote areas, or in a tunnel, or during an electronic outage.

In 2018, hundreds of offenders in SA were left unmonitored following an electronic failure blamed on a nationwide Telstra outage.

Who monitors these devices?

This depends on each state and territory and how they outsource the work.

The company providing equipment to the NT said it also provided monitoring of offenders in the community.

However, the monitoring only shows where a person is and not what they are doing. Essentially, an offender is just a dot on a map.

The only reasons authorities would be alerted to their whereabouts is if they have not charged the device correctly, or if they enter an area they are restricted from by virtue of a court order.

An alert will also be sent out if the offender is not where they should be during a court-imposed curfew, or if the wearer of the device tries to remove it.

In all of those instances, police are notified.

NT authorities said the alleged Darwin gunman was released on parole in January with 23 strict conditions.

"While on his parole the offender received one minor breach for breaking his curfew, for less than an hour, and was subject to a 14-day custodial sentence as a result," corrections commissioner Scott McNairn said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 41 seconds 1 m 41 s Criminologist speaks about ankle bracelets

Why is electronic monitoring used?

Electronic monitoring is designed to increase community protection.

They are part of a suite of options used by a court, and generally come with a list of other restrictions the wearer must abide by.

"So, it's not about reducing the number of people in custody and dealing with the overcrowding problems," Dr Martinovic said.

"A person on parole, a person on an extended supervision order, a person on a community-based order is added electronically monitoring as a condition or their [court] order.

"In the past, they didn't have that condition, they were running around free and nobody knew where they were."

According to Dr Martinovic, the use of electronic monitoring devices has increased by a whopping 150 per cent between 2016 and 2019.

That uptake is much higher than elsewhere in the world.

However, Australia was late to the technology and now catching up with the rest of the developed world.

Driving that increase is a stronger push towards community safety.

Some civil liberties groups have previously cautioned against the overzealous use of the devices and said the courts needed to think thoroughly about when to use the devices.

