Attorney-General George Brandis has told Senate Estimates a letter from Sydney siege gunman Man Haron Monis asking about the legality of contacting Islamic State did not indicate he would be a threat to the community.

The inquest into the deadly December siege heard Monis sent a letter directly to Senator Brandis in early October last year.

"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 [sic]," Monis wrote in his letter, under the name of Sheik Haron.

"Please advise me whether the communication is legal or illegal."

The letter was received in Senator Brandis's parliamentary office, before being sent to his Department's National Security Law and Policy Division.

In Senate Estimates, Senator Brandis was questioned as to why the letter did not trigger alarm bells within his office or the department.

He said he did not know how often letters asking similar questions were sent to his office.

"I'll take that on notice, I'll find out how often correspondence of that kind has been received," Senator Brandis said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 8 minutes 54 seconds 8 m The bizarre, delusional world of Man Haron Monis.

"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.

"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."

Almost a month later, Acting Assistant Secretary of the Division Karen Horsfall replied to Monis.

"The Islamic State is listed as a terrorist organisation," she wrote.

"Under the Criminal Code, it is an offence to direct the activities of, be a member of, recruit for, provide training to, receive training from, provide funds to or receive funds from or provide support to a terrorist organisation.

"The Attorney-General's Department does not provide legal advice to the members of the public and cannot address your specific question about the legality of corresponding with Caliph Ibrahim.

"Should you have further concerns about the legality of your proposed actions, you might like to consider obtaining professional legal advice."

'No reason to believe Monis a person of concern at time'

Senator Brandis maintained department staff did not see the letter as a threat.

"There was no reason to believe that any member of the Attorney-General's Department staff would have known that Monis, or Haron as he signed himself, was a person of concern at that particular time," he said.

"If I was to send a letter to Islamic State, you don't see concern about that?" independent senator Jacqui Lambie asked.

Senator Brandis responded that such judgments were easier made in hindsight.

Monis's letter was received just weeks after Prime Minister Tony Abbott announced Australia's terror threat level was being lifted to "high".

Less than two months later, Monis stormed the Lindt cafe in Sydney.

Labor senator Jacinta Collins raised comments from the Prime Minister upon raising the threat level, where he called on all Australians to report suspicious activity to the National Security Hotline.

Shadow Attorney-General Mark Dreyfus said Monis's reputation was well known before he sent the letter, which 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 ISIL that I think takes it well out of the range of routine," he said.

"A letter from a man on bail for serious violent offences, 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."