Attorney-General George Brandis says he was only informed on Monday that letters between his department and the Sydney siege gunman were not given to a high-level inquiry into the incident.

Last week, Foreign Minister Julie Bishop told Parliament that both the letter, sent by Man Haron Monis last October, and the reply from the Attorney-General's Department were provided to a joint New South Wales-Commonwealth inquiry into the fatal siege.

But Ms Bishop told Parliament on Thursday that was not the case, saying she had relied on false advice from the deputy secretary of the Attorney-General's Department, Katherine Jones, in Senate estimates.

Senator Brandis issued a statement saying he did not know that the letters had not been passed on until Monday.

"I then had a meeting with the secretary of my department [Chris] Moraitis, and asked him to conduct an urgent inquiry within the department to establish the facts," the statement said.

"My office received Mr Moraitis' report early this afternoon, which confirmed that the Monis letter and the department's reply had not been provided, contrary to the earlier departmental advice."

He said Ms Bishop was told shortly before Question Time, and revealed the misinformation in Parliament.

"The letter and reply were not provided to the review due to an administrative error in the Attorney-General's Department," Ms Bishop said.

"Ms Jones has written to the Legal and Constitutional Affairs [Committee] to correct her evidence."

Ms Bishop said the Attorney-General's Department had since undertaken a review of the mistake to ensure all officials who relied on Ms Jones's evidence could have the opportunity to correct the record.

"I'm advised that both letters have now been referred to officials at the Department of Prime Minister and Cabinet, who were responsible for the Sydney siege review," she said.

"The Secretary of Prime Minister and Cabinet Mr Michael Thawley has written to the Prime Minister today to inform him that the letter in response would've made no difference to the outcome of the review."

Manager of Opposition Business Tony Burke has questioned the timing of the admission — after Question Time, on the last sitting day before the weekend.

"There's no way in the world that the first moment they had to correct the record was at the end of Question Time," Mr Burke said.

"All of them need to make clear why they wanted to wait until Question Time had finished and scrutiny had finished."

Mr Burke said the admission flew in the face of the Government's fiery response to questioning by shadow attorney-general Mark Dreyfus over the issue last week, describing it as playing politics with a tragedy.

He questioned what Ms Bishop and Senator Brandis were doing in the lead-up to the admission — including the Attorney-General, who is also the Arts Minister, reciting poetry in Senate estimates last night.