A letter from the Lindt Cafe gunman to the Federal Attorney-General was not included in a review of the Sydney siege because it was on the second tab of a spreadsheet and was missed, according to a senior official in the Attorney-General's department.

A Senate committee is investigating the circumstances surrounding the siege and the handling of the letter, sent directly to George Brandis less than three months before Man Haron Monis took hostages in the Sydney cafe.

In the letter Monis asked for advice about whether it was legal to contact 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," the letter said.

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

Prime Minister Tony Abbott announced an urgent internal review to investigate the circumstances leading up to the deadly shootout and the contact Monis had with authorities.

Senator Brandis originally said the letter was included as part of the internal review process, but earlier this month Foreign Minister Julie Bishop told parliament the document had not been handed over because of an "administrative error".

Attorney-General's Department deputy secretary Tony Sheehan told today's committee hearing the letter was identified by his staff.

But he said the letter was missed when all the material relating to the inquiry was collated to hand over to the Department of Prime Minister and Cabinet for the internal review.

"They [department staff] did not realise there was a second tab to that spreadsheet," he told the hearing.

"So they collated the documents and provided the documents to PM&C [Prime Minister and Cabinet] appropriately, but didn't know that there was this document that was in scope on another page of the spreadsheet.

"It was a human error essentially and that wasn't detected through the clearance process that occurred. As a result, although the document had been identified, it wasn't then passed to the review."

After the error was made public, Senator Brandis issued a statement saying the secretary of Prime Minister and Cabinet Michael Thawley had reviewed the letter and it "would've made no difference to the outcome of the review".

When the contents of the letter were first revealed Labor criticised Senator Brandis for failing to refer it to police, arguing it contained suggestions Monis was involved with terrorist activity.