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Sydney siege: Prime Minister Tony Abbott asks why 'madman' was not on ASIO watchlist

Prime Minister Tony Abbott has described the gunman responsible for the deadly Sydney siege as a "madman", and conceded systems to monitor and prevent the attack were not adequate.

This morning Mr Abbott acknowledged failures in the system.

"The system did not adequately deal with this individual, there's no doubt about that," he told the AM program.

"We've got to be constantly asking ourselves 'Is this the best we can do?'

"And frankly, we've got to always be better at this because if we aren't good at this, our people suffer.

"The tragedy of this atrocity is that two delightful Australians, two very decent people are dead.

"Others are injured. Others are traumatised because of a madman who was roaming our streets."

Mr Abbott said the gunman was well known to state and federal police and the domestic spy agency ASIO.

The Prime Minister also revealed his own nagging thought: could the siege have been prevented?

He said Cabinet's national security committee had the same concern.

However, Mr Abbott conceded Man Haron Monis was not on a security watchlist, despite his long criminal history and known "infatuation with extremism".

Mr Abbott said the public had a right to know how someone with a long and chequered history was not on the appropriate watchlists.

The Prime Minister said he wanted answers.

"How can someone like that be entirely at large in the community?" Mr Abbott said.

"These are questions that we need to look at carefully and calmly and methodically to learn the right lessons and to act upon them.

"That's what we'll be doing in the days and weeks ahead."

The Prime Minister acknowledged Monis still might not have been stopped, even if he had been closely watched.

"Even if this individual, this sick and disturbed individual, had been front and centre on our watchlists, even if this individual had been monitored 24 hours a day, it's quite likely, certainly possibly, that this incident could have taken place because the level of control that would be necessary to prevent people from going about their daily life would be very, very high indeed," he said.

Mr Abbott again reinforced the attack was an isolated one.

"This was the act of a deeply unstable person with a long history of violence and mental illness.

"This was the act of someone who was way beyond any mainstream - any mainstream - and has been rightly, absolutely repudiated by all the communities of Australia."

Monis 'wasn't a candidate' for surveillance

However, the question of around-the-clock surveillance had its own challenges according to former army officer Dr Rodger Shanahan, a non-resident Fellow at the Lowy Institute for International Policy.

"It's all very difficult to keep 24 hours a day surveillance on somebody," Dr Shanahan said.

"Even if you did, if he had a sawn off shotgun in a bag, as appears to be the case, and he just travelled from his house on public transport going into the city, which is a very innocuous thing to do, how are you to know if he's not part of any kind of group or organisation or has done little or no planning for this, how are you to know that he's not just going into the city?

"And how could you have stopped it, even if you saw him getting on the public transport?"

Dr Shanahan also suggested the gunman's criminal history did not automatically make him a candidate for surveillance.

"Well as far as we know, the violence that he was said to have done, that's not politically motivated violence," he said.

"You know, there's 40 sexual assaults from a decade ago.

"There's an attack on his ex-wife, [he] was accused of being an accessory to murder.

"Those are civil instances; they're not politically motivated violence.

"Certainly the letters that he sent to the families of Australian soldiers had a political connection, or a direct political relevance, but his violent acts as far as we know just seemed to be part of criminal behaviour, not political behaviour."

Despite the Sydney siege, the Prime Minister remained hopeful.

"Every Australian is obliged to treat other Australians, every other Australian with respect," Mr Abbott said.

"And this mutual respect, this notion that we should treat others as we would be treated ourselves, this is at the heart of our society.

"It's at the heart of all decent societies. And I think all Australians, regardless of their religion, get that."