A major transnational police operation has led to dozens of arrests in New Zealand and more than 900 charges being laid against criminal organisations.

Search warrants have been executed across the North Island, including in Auckland, Waikato, Bay of Plenty and Wellington, with more arrests expected in the coming days.

Raids were also carried out overnight in the United States, Europe and Australia.

At the heart of the operation - which is being referred to as "Operation Ironside" in Australia -  is an FBI trojan horse application called AN0M that allowed authorities to track communications by organised crime members in real time.

So what is AN0M and how does it work?

The app

The application was an encrypted platform run by law enforcement agencies.

It was sold on the black market and people could only get access to it if they were referred to a seller through an existing criminal user, or had a distributor who could vouch they were not working for law enforcement, News Corp reported.

The platform included a mobile phone stripped of the majority of its functions and could only do three things: send messages, distort voices and take videos.

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According to News Corp, in Australia handset and subscription packages cost between AU$1500 and AU$2500. Customers did not need to sign up with their real names and some paid in Bitcoin.

The app came preloaded onto handsets and was hidden behind a fully functional calculator icon.

The app was activated when a pin was entered into the calculator. It offered end-to-end encryption, self-expiring messages and also had a "duress pin" which when entered would wipe the device of all information if necessary.

Police see everything

Unbeknownst to those using the app, police could see messages sent via the app and were storing them on their own servers. They could also undistort voice messages sent with AN0M.

However, due to the fact criminals didn't need to reveal their identity when signing up for the app, it was often an arduous process for police to figure out exactly who was sending the messages.

"The time it took to attribute devices could sometimes become quite deflating, especially when you knew how significant they were. Some attributions could take hours, some days and some weeks...even months,'' one senior police team leader told News Corp.

Police said matching users with the devices required digital detective skills and sometimes involved officers staking out places mentioned in the messages and seeing who appeared.

A spokesperson for Australian Federal Police told News Corp all material collected was used "to glean any insight" possible into the user.

"These clues, coupled and compared with other sources of information, enable us to put a person to each device. Establishing the identity of the users is crucial to disrupting the criminal activity they facilitate using the devices," the spokesperson said.

The sting is being described as the biggest ever in the Southern Hemisphere, with police in Australia saying they have foiled 21 murder plots and uncovered the trafficking of billions of dollars worth of drugs into Australia. 

In New Zealand 37 search warrants have so far been executed resulting in the arrest of 35 people and more 900 charges being laid.

Police said more search warrants will be executed on Tuesday with more arrests expected.

National Organised Crime Group Director Detective Superintendent Greg Williams said New Zealand police got access to information coming from AN0M in early 2020.

"We had 57 devices being used here in New Zealand," he told media on Tuesday.

"Every single one of those devices was being used for criminal means."