Australian PM asks why Sydney siege killer was not on watchlist

Australian Prime Minister Tony Abbott on Wednesday questioned why the gunman responsible for the tragic Sydney siege was not on a security watchlist.

The gunman who died in the Martin Place siege had a history of violence and was well known to state and federal police and the domestic spy agency Australian Security Intelligence Organization (ASIO).

Abbott said Man Haron Monis was not on a security watchlist, despite his long criminal history and known "infatuation with extremism" and the public had a right to know why he wasn't being closely monitored.

"How can someone like that be entirely at large in the community?" Abbott told ABC radio. "These are questions that we need to look at carefully and calmly and methodically to learn the right lessons and to act upon them.

"We've got to be constantly asking ourselves 'is this the best we can do?' That's what we'll be doing in the days and weeks ahead. "

Abbott said Monis still might not have been stopped, even if he had been closely watched. But he also said the attack was an isolated one from a deeply unstable person with a long history of violence and mental illness.