Australian terror suspects are younger and acting faster than ever before, NSW Police Deputy Commissioner Cath Burn says.

Ms Burn says counter-terror investigations across NSW have soared by 40 per cent in the past year, with more than 40 people now before the courts on terrorism or related charges.

The number of terror suspects aged between 15 and 25 has also tripled in the past two years, she told a national policing summit in Sydney on Wednesday.

"The normal modus operandi that we're seeing is rudimentary weapons, solo or small groups of people, (and) really rapid onset of the intent to do something turning into actually doing something," Ms Burn told police forces and counter-terror experts from across Australia.

The rise of Islamic State (IS) had completely changed the terror landscape, she said.

"Once IS started to take more ground in the conflict zone in Iraq and Syria, and a caliphate was declared, that's when we really saw the change in the tempo," Ms Burn said.

"Since then we've now prevented nine imminent attacks, and we've seen three, in Numan Haider in Melbourne, Man Monis in Martin Place and the Curtis Cheng murder in Parramatta.

"The ones we've seen prevented have been completely different from the large-scale attacks."

Ms Burn's address followed the overnight arrest of a 17-year-old for allegedly planning a terror attack in Sydney.

Outside the conference earlier in the day, NSW Deputy Premier Troy Grant said the teen's arrest was sombre news.

"It just emphasises to me the real risk that we face here, and I'm very grateful of the work of the police and the partnerships they have, that they were able to make that arrest before anything happened," Mr Grant said.

In a keynote address on Wednesday morning, Justice Minister Michael Keenan told the conference the threat of radical Islamic terror is at an unprecedented scale and showing no sign of abating.

"We need to accept that this threat from radical Islam will continue to be one of the constants in our national security environments for the foreseeable future," he said.

Mr Keenan said the Orlando nightclub massacre at the weekend drove home this grim reality.

"Attacks like this are not just an assault on the victims, they're an assault on our freedom itself," he said.

"The ideology that drives this perversion of religion hates everything that western democracy stands for, and it believes that violence is the best way to achieve their medieval vision."

Mr Keenan said the Sydney teen's arrest on terror charges made plain the need to pump money into crime-fighting intelligence and surveillance.

He said around 110 Australians were fighting or engaging in terrorist groups in Middle East conflict zones, with at least 50 killed fighting alongside combatants.

Around 190 people in Australia were being investigated for supporting terror organisations, with 175 Australian passports cancelled.