U.S. and Australian authorities hacked into an app used by criminals to read millions of encrypted messages, leading to hundreds of arrests of suspected organised crime figures in 18 countries, Australian officials said on Tuesday.

"Operation Ironside" by Australian police and the U.S. Federal Bureau of Investigation ensnared suspects in Australia, Asia, South America and the Middle East involved in the global narcotics trade, the officials said.

Australian Prime Minister Scott Morrison said the operation "struck a heavy blow against organised crime – not just in this country, but one that will echo around organised crime around the world".

"This is a watershed moment in Australian law enforcement history," Morrison told reporters in Sydney.

Australian Federal Police Commissioner Reece Kershaw said police raids in 18 countries netted hundreds of suspects. Europol and the FBI said on social media they would hold news conferences later on Tuesday.

Australia said it had arrested 224 people, including members of outlawed motorcycle gangs, while New Zealand said it had detained 35 people.

The operation, which was conceived by Australian police and the FBI in 2018, saw officials in the United States take control of the ANOM messaging app, which is popular with organised crime networks.

When an Australian underworld figure began distributing customised phones containing the app to his associates as a secure means to communicate, police could monitor their messages. The gangs believed the system was secure because the phones did not have any other capabilities - no voice or camera functions were loaded - and the app was encrypted.

"We have been in the back pockets of organised crime," Kershaw said at the same media briefing. "All they talk about is drugs, violence, hits on each other, innocent people who are going to be murdered."

The messages were brazen and there was no attempt to hide behind any kind of code, he said.

"It was there to be seen, including 'we’ll have a speedboat meet you at this point', 'this is who will do this' and so on."

Kershaw said the Australian underworld figure, who had absconded from the country, had "essentially set up his own colleagues" by distributing the phones and was a marked man.

"The sooner he hands himself in, the better for him and his family," he said.

One murder plot that authorities got to know of involved plans to attack a cafe with a machine gun, while a family of five was also targeted. Authorities said they were able to prevent these attacks.

Executing Australia's largest number of search warrants in one day, police on Monday seized 104 firearms as well as almost A$45 million ($34.9 million) in cash.

A total of 525 charges have been laid but authorities expect more in the coming weeks.