Brandis plays down Monis letter 'alarm bells'

Attorney-General George Brandis has played down the significance of a letter he received from Man Haron Monis asking whether it was legal to contact Islamic State extremists, two months before he stormed Sydney's Lindt cafe.

Senator Brandis told a Senate estimates hearing overnight that department staff did not consider the letter threatening, despite it being received weeks after Australia's terror threat level was raised to high.

"There was no reason to believe that any member of the Attorney General's department staff would have known that Monis ... was a person of concern at that particular time," he said.

Shadow attorney general Mark Dreyfus maintains the letter should have prompted further investigation.

"A letter from a man on bail for serious violence offences, who had 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," Mr Dreyfus told the ABC radio.

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