A review into the Sydney siege was not provided with a letter gunman Man Haron Monis sent to the federal attorney-general because someone didn't realise it was on a second tab of a spreadsheet.
A Senate inquiry into the oversight has also heard that a senior bureaucrat, who told another parliamentary hearing the letter had been sent, only realised the mistake when she sat down for lunch with a colleague.
Katherine Jones, deputy secretary of the Attorney-General's Department, said her error became obvious during a meeting with Allan McKinnon from the Department of Prime Minister and Cabinet in late May - two days after she appeared before a Senate estimates hearing.
After reviewing her evidence over the weekend, the department checked back through its records.
Secretary Chris Moraitis defended the time it took to correct the record, despite the review indicating it hadn't received the letter on Monday June 1.
Attorney-General George Brandis was advised the same day but parliament was not informed of the "administrative error" until the Thursday and then only after question time.
"I wanted the correction to be ... the absolutely correct correction," Mr Moraitis told senators in Canberra on Friday.
Labor senator Jacinta Collins asked why the department didn't just immediately ring the review and ask whether they had the letter.
"Then it would have been case closed, advice to the minister, correction on the Monday," she said.
Deputy secretary Tony Sheehan said they did that but also wanted to know what happened at the department's end.
Mr Moriatis insisted he was still more "confused than clear" on Monday night when he spoke to Senator Brandis.
The October 2014 letter, in which Monis asked the attorney-general for legal advice, was discovered in a search for relevant material but wasn't passed on to a joint federal-NSW inquiry that considered how agencies responded to the siege.
"It was a human error essentially," Mr Sheehan told senators.
Other documents on the tab that were not sent to the inquiry included two more letters from Monis to other politicians, another to a government agency and one to a private citizen.
"He wrote literally hundreds of letters to ministers, parliamentarians," Ms Jones said.
Officials from the Department of Prime Minister and Cabinet will face questioning on Tuesday night.
Monis and two hostages died in the Lindt Cafe last December in Sydney's Martin Place after a 17-hour siege.
Shadow attorney-general Mark Dreyfus said the evidence showed the government was committed to "cover-up and deception".
"While Mr Abbott beats his chest about being tough on national security, his own government can't even recognise a terrorist when he writes to the attorney-general saying he wants to contact ISIS," Mr Dreyfus said in a statement.
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