New emails show Prime Minister Tony Abbott may have known Parliament had been misled about the Sydney siege gunman three days before the government corrected the record.

Labor has seized on the documents as more proof of a "cover-up" and demanded Mr Abbott explain what he knew and when.

Man Haron Monis wrote to Attorney-General George Brandis in October 2014 asking if it was legal for him to contact the leader of Islamic State. An official drafted a routine reply, despite the national terror alert level having earlier been lifted to 'high'.

In December last year Monis walked into the Lindt cafe in Martin Place armed with a gun and Islamist flags, triggering a siege that led to the deaths of two hostages.

But a joint Commonwealth-NSW inquiry into the siege never examined his letter to Senator Brandis because an official didn't realise it was on the second tab of a spreadsheet.

Foreign Affairs Minister Julie Bishop told Parliament - based on mistaken evidence from an official - that the letter had been sent to the inquiry.

She eventually corrected the record but waited until the end of a sitting fortnight to do it, meaning the government avoided scrutiny in question time about its handling of the case.

A Senate committee has already discovered Senator Brandis and Mr Abbott's office knew about the letter at the start of the sitting week.

However, the new emails show a 'PPQ' - a briefing about possible lines of attack on the issue from Labor in question time - was prepared for Mr Abbott on Monday, June 1 - three days before Ms Bishop made the correction on June 4.

The brief - prepared by the second-in-charge at the Department of Prime Minister and Cabinet - explicitly stated the inquiry was never given the Monis letter.

The brief was emailed to an unnamed official at 11.50am. Question time commended at 2pm.

However Labor did not ask any questions about the issue that day, so Mr Abbott did not need to use the brief. It is unclear whether he read it.

A spokesman for Mr Abbott did not answer questions about whether the Prime Minister had read the question time briefing.

Labor's leader of the opposition in the Senate, Senator Penny Wong, accused the government of a cover-up.

"It's time for the Prime Minster to tell the truth about what happened here," she said.

"The Prime Minister cannot continue to duck questions about the part he and his office played in this cover-up. The Prime Minister is accountable for this cover-up."

Senator Wong also said ministers have resigned over misleading the Parliament in the past.

A spokesman for Mr Abbott said a "thorough review" was undertaken by the Attorney-General's Department once it became clear in July that the letter had not been sent.

"As soon as that review was completed, the record was corrected by the Attorney-General and by the Foreign Minister."

The spokesman also said Department of Prime Minister and Cabinet secretary Michael Thawley - one of the two bureaucrats who conducted the inquiry - has noted the letter would have made no difference to its findings even if it had been seen.

TIMELINE

October 7, 2014: Man Monis writes to Attorney-General George Brandis

December 16, 2014: Sydney siege ends with the killing of two hostages and the death of Monis

December 17, 2015: Tony Abbott announces a joint Commonwealth-NSW inquiry

January 31, 2015: Inquiry is delivered to Mr Abbott

February 2, 2015: Attorney-General's Department realises Monis letter never went to inquiry

February 2, 2015: Department contacts review team by phone but told the document did not need to be provided because the review had been concluded

February 4: Inquiry authors sign off report for printing

May 27, 2015: Department official tells Senate estimates hearing the letter had been sent

May 28, 2015: Julie Bishop quotes that official's evidence to deflect Labor attack about why the Monis letter didn't raise red flags

June 1, 2015: Prime Minister's Office and Attorney-General told the letter did not go to the inquiry. Question time briefing prepared for Tony Abbott explicitly states the inquiry was never given the Monis letter

June 4, 2015: Julie Bishop corrects the record at the end of the last question time for that sitting fortnight

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