Australian Prime Minister Tony Abbott admitted on Wednesday that security measures are not enough to prevent an attack similar to the one that a hostage taker carried out in a Sydney cafe.

Abbott called the gunman in the fatal Sydney siege, Man Haron Monis, a "madman" and acknowledged cracks in the nation's security system.

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The prime minister said the government has to improve security measures because if they are not adequate, then Australians suffer.

He found the the loss of the lives of two Australians with several others injured unacceptable. He added others are traumatized because the gunman was allowed to roam Sydney's streets.

He said state and federal police knew the attacker well, as did the state spy agency ASIO.

Abbott did concede Monis did not appear in any security watchlist, despite his chequered past and known belief in extremism.

He said citizens had a right to know why someone with Monis' history was not on any watchlist.

The prime minister said he was working on answers and there are questions that needed to be studied to learn the right lessons.

He acknowledged security forces might not have stopped Monis, even if he had been under close scrutiny. He said the attack would have been still carried out because the resources needed to check on people who go on with their daily activities would be too much.

Abbott noted the attack was executed by a lone gunman -- the act of a deeply disturbed person with a long history of violence and mental illness. 

He said the gunman was acting beyond mainstream society and he has been rightly rejected by Australian communities. 

TagsSydney Hostage