Criminals were lured into using the FBI-run ANOM app, where their messages were monitored.
Law enforcement agencies say they have arrested hundreds of criminals around the world in a three-year operation, using a secure messaging app run by the American FBI.
The operation, jointly conceived by Australia and the FBI, saw the app ANOM secretly distributed among criminals, allowing police to monitor their conversations without their knowledge.
It has led to arrests in 18 countries.
They include suspects linked to the mafia and organised criminal groups.
Drugs, weapons and cash have also been seized.
Australia said it had arrested 224 people as a result of the operation, and had acted on 20 “threats to kill”, potentially saving the lives of a “significant number of innocent bystanders”.
The country’s prime minister called the sting a “watershed” operation that had hit criminal gangs globally.
“[It] has struck a heavy blow against organised crime – not just in this country, but one that will echo around organised crime around the world,” Scott Morrison said in a press conference.
New Zealand, which detained 35 people, called the operation the “world’s most sophisticated law enforcement action against organised crime to date”.
The FBI and Europol will present more details later on Tuesday.
New Zealand police said that after the FBI had dismantled two other encryption services, it began operating its own encrypted device company called ANOM.
Devices with the chat app were distributed in the criminal underworld.
Australian police said the devices were initially used by alleged senior crime figures, giving other criminals the confidence to use the platform.
Fugitive Australian drug trafficker Hakan Ayik was key to the sting, having unwittingly recommended the app to criminal associates after being given a handset by undercover officers, they said.
“You had to know a criminal to get hold of one of these customised phones. The phones couldn’t ring or email. You could only communicate with someone on the same platform,” the police explained.
Officers were able to read millions of messages in “real time” describing murder plots, mass drug import plans and other schemes.
“All they talk about is drugs, violence, hits on each other, innocent people who are going to be murdered, a whole range of things,” said Australian Federal Police commissioner Reece Kershaw.
Commissioner Kershaw said Ayik was a marked man and should turn himself in.
Commissioner Kershaw said that in Australia, police had been able to intercept drug operations and prevent incidents such as mass shootings.
The more than 200 arrests in the country included members of outlaw motorcycle gangs, Australian mafia groups, Asian crime syndicates and serious and organised crime groups.
Australian police have also seized three tonnes of drugs and A$45m (£25m; $35m) in cash and assets.
Authorities said their sting, which they called Operation Ironside, was the nation’s largest police operation and involved 4,000 police officers.
Some 9,000 police officers were involved worldwide.
“Knocking out their communications has been a key part of us disrupting the organised crime,” Commissioner Kershaw said.
He said the app access had given law enforcement “an edge that it had never had before”, but added the platform was just one of many messaging apps favoured by organised crime gangs.
New Zealand police said they had laid more than 900 charges in relation to the 35 people arrested.
About NZ$3.7m (£1.9m, $2.7m) of assets were seized.
“We believe the termination of these operations will have a significant impact on New Zealand’s organised crime scene,” National Organised Crime Group Director Detective Superintendent Greg Williams said.
In 2020 major crime figures were among over 800 Europe-wide arrests after messages on another encrypted app, EncroChat, were intercepted and decoded.