Australian authorities are struggling to crack the highly secretive "app of choice" for terror plotters worldwide, partly stifled by the app creators' refusals to push back on jihadi infiltration.

Telegram, an encrypted messaging service and platform for hosting information-sharing "channels", has increasingly featured in terror acts.

Reports from London on Monday suggested a terror cell from Barking, who were recorded discussing a plot identical to the Borough Markets car and knife attack, had discussed using Telegram.

The fanatics behind the Berlin and Nice truck attacks had communicated via the app, as had the Paris attackers who killed 130 people in November 2015.

In Australia, the group charged over the shooting of Curtis Cheng in Parramatta communicated via a similarly encrypted app, WhatsApp, in a group chat they labelled The Bricks.

ASIO director-general Duncan Lewis said use of encrypted communication by terrorism targets was "widespread" and remained "a particular area of concern".

"Rapid technological change continues to provide ... new tools to conceal their activities from security and law enforcement agencies," he told a Senate estimates last month.

In a sign of the times, the number of telephone intercept warrants taken out by the NSW Crime Commission had fallen by almost 60 per cent in two years, a result of what the agency called an "exponential increase" in encrypted technology.

While not unbreakable, apps such as Telegram required considerable resources to penetrate.

Project Muskwood, an Australian Criminal Intelligence Commission investigation into encrypted apps, had had some success in getting partner countries to work with manufacturers based in their countries, acting chief Nicole Rose said.

A recent report by the US-based Middle East Media Research Institute called Telegram "the app of choice" for Islamic State members and supporters.

While most apps only offered one-to-one messaging, Telegram had public and private channels where groups shared propaganda, attack tutorials, kills lists and recruitment drives.

Increasingly, attacks had been virtually directed by overseas jihadists who provided minute-by-minute directions to recruits via Telegram.

It had become a "fertile and secure arena" for IS and there "appears to be no way in to monitor it", MEMRI said.

For data not covered by end-to-end encryption, Telegram distributed its infrastructure across the globe to make it hard for any one country to get a court order for data.

"Thanks to this structure, no single government or block of like-minded countries can intrude on people's privacy and freedom of expression," said Telegram, founded by Russian tech guru Pavel Durov and his brother Nikolai.

Europol head Rob Wainwright said last month that Telegram was doing far less than other social media companies to tackle terrorism.

Frustrated authorities in the US and Britain wanted encrypted apps to be required to provide "backdoor" access to security agencies.

However, after the Edward Snowden case, the idea was unpopular with everyday users, such as the 1 billion users of WhatsApp.

In a March blog post titled Don't Shoot the Messenger, Telegram said it regularly removed IS channels but the blame for spreading terrorists' messages lay with the media.

It said a ban on end-to-end encryption would only make terrorists build their own apps or find other methods to communicate.

"The only thing government-mandated backdoors can achieve is make mass surveillance possible again and expose your private data to hackers or corrupt officials," the post said.

Charles Sturt University academic Nick O'Brien, a former Scotland Yard counter-terrorism chief, said Australian authorities would have to learn to deal with apps like Telegram.

He said police had had to invest more money into IT systems that could crack encryptions and rely more on old-school human intelligence.

"It's almost come full circle where they have to go back to human sources," he said. "It's expensive, it's risky and it's resource-intensive."

Often, the only option for police was to infiltrate apps individually. In one investigation into an Auburn teenager who was trying to source a gun to plot an Anzac Day attack in 2016, officers went undercover to talk to him on a secure app.

A spokesman for Attorney-General George Brandis said the use of encrypted apps by terrorists was an "increasing challenge" and the government was working with tech companies to "facilitate and support serious investigations".

"The private sector has an important role to play in assisting police and security agencies where terrorists use encryption to conceal their activities," he said. 

"The government continually reviews the adequacy of our legislation to make sure our law enforcement and security agencies have the powers they need."