Australia's A-G says "ISIL letter" from Sydney siege gunman not suspicious

Australia's Attorney-General has said a letter written to him by the Sydney siege gunman asking about the legality of contacting terrorist group Islamic State was not "obviously threatening."

Man Haron Monis, who held 18 hostages in a city cafe for 17 hours in December, sent a letter to Australia's Attorney-General George Brandis in October which was received by Brandis' parliamentary office before being sent to his department's National Security Law and Policy Division, according to an Australian Broadcasting Corporation (ABC) report on Thursday.

Addressing a Senate Estimates hearing, Brandis said the tone of the letter did not alert his department to any danger Monis might pose to the community.

Just weeks before the letter was received, Australia's terror threat level was raised to "high", implying a terror attack was likely.

"I'm bound to say the tone of the letter is not obviously threatening, nor does the letter apparently contain any endorsement or indication of favourability towards Islamic State," Brandis said on Wednesday.

"It merely, in a neutral tone, asks a question about whether to communicate with this individual by making comments and asking questions is legal or illegal."

The letter questioned whether it was a crime to contact the Islamic State.

"I would like to send a letter to Caliph Ibrahim, the leader of the Islamic State, in which making some comments and asking some questions," Monis wrote. "Please advise me whether the communication is legal or illegal."

Weeks later, Acting Assistant Secretary of the Division Karen Horsfall replied to Monis, outlining the terrorist status of the Islamic State and advising Monis to contact a lawyer if he wanted legal advice.

Shadow Attorney-General Mark Dreyfus said Monis was well known by authorities before he sent the letter and the correspondence should have sparked an investigation.

"It's what's known about Mr Monis at this time already by the Commonwealth and the fact of him proposing communication with Islamic State of Iraq and the Levant (ISIL) that I think takes it well out of the range of routine," Dreyfus told the ABC on Thursday.

"A letter from a man on bail for serious violent offenses, who' d been in litigation with the Commonwealth in the High Court, asking about communication with the head of ISIL, should not have been treated as routine."

"It's something that should have been referred."

Less than two months later, Monis stormed the Lindt cafe in Sydney and held hostages in a 17-hour siege. He eventually shot the manager of the cafe, sparking a police shootout where Monis and another hostage were killed.

An inquest into the Sydney siege began taking evidence on Monday and is expected to run into 2016. Endi