Global crime-fighting agencies have come clean on a secret FBI-developed phone sting that successfully spied on some of the world's most ruthless gangs after an anonymous tech geek blew the whistle on the fake devices months ago.

After five years of clandestine planning, coordinated raids yesterday across 16 countries, including New Zealand, have resulted in more than 800 arrests, tons of drugs seized, and millions of cash uncovered.

Kiwi police officers carried out raids across the North Island, resulting in 35 arrests, including senior members of the Comancheros, Mongrel Mob and Head Hunters gangs. More than 900 charges have been laid - and more are expected in court today.

They have allegedly been caught in "sting of the century" Operation Trojan Shield - an elaborate international sting that began with the FBI developing its own fake phones and encrypted messaging service, ANOM.

It lured criminals into thinking they could openly discuss drug deals and shipments – even, allegedly, contract killings - without being tracked.

Around 12,000 ANOM encrypted devices and services were sold between some 300 criminal syndicates operating in more than 100 countries, including Italian mafia, Asian crime rackets, outlaw motorcycle gangs, and various international drug trafficking rings, officials say.

Criminals paid a subscription fee for the supposedly secret service – amounting to about A$1700 in Australia, according to US court documents seen by the Herald.

However, little did they know that authorities could access every conversation. It's alleged that agents catalogued a staggering 27 million messages.

So why did the authorities feel the need to trumpet just how they took down so many major crime players yesterday?

"Part of it has to do with legal authorities," Australian Federal Police (AFP) Commissioner Reece Kershaw said yesterday in a press conference alongside Prime Minister Scott Morrison on Special Operation Ironside, as it was dubbed across the Tasman.

"And also just the scale and the speed of the organised crime, and with the threats to life and the harm that was being done, it was an operational decision that was made jointly with all the international partners including the Europol operation taskforce which is run out of The Hague."

He also confirmed that there was a "legal timeframe for this operation" in which Australia worked alongside the FBI to provide the "technical capability" to decrypt the chatter.

The intercepted messages were "very brazen", Kershaw said, and nothing like police had ever seen before.

There was no attempt to hide behind any codes or disguised language.

"Let me be clear, when you get access, and it'll come out in court, you'll see that all they talk about is drugs, violence, hits on each other, innocent people who are going to be murdered, a whole range of things... We haven't seen it done like that."

But the operation could potentially have been compromised months ago.

On March 29, an anonymous blogger writing as 'canyouguess67' correctly called out the "ANOM encrypted scam".

The warning, which included a lengthy technical explanation as to why ANOM should not be trusted, concluded: "Stay away from ANOM if you value your privacy and safey [sic]. They are compromised, liars and your data is running via USA – passed on to law enforcement and other entities".

The blog, 'ANOMEXPOSED', has since been taken down.

It's not clear whether the online post came to the attention of either criminals or law enforcement agencies – or whether it played any role in yesterday's coordinated action.

New Zealand police nonetheless were delighted with the arrests.

"This is a stunning piece of work," National Organised Crime Group director Detective Superintendent Greg Williams said yesterday.

"It's a great day for New Zealand.

"We believe the termination of these operations will have a significant impact on New Zealand's organised crime scene."