The Attorney-General's department was "ducking for cover" after losing evidence in the review of December's fatal Sydney siege, according to senior Commonwealth bureaucrats.
But the boss of AGs told a parliamentary committee on Monday that "running around like headless chooks" was a more accurate description of his department's conduct.
A series of emails between senior public servants in the Prime Ministers' Department shows the frustration they felt at the counterparts in AGs after letters from the killer Man Haron Monis were lost, and misleading answers given to the Parliament over the affair.
The emails, provided by the public service to a committee investigating the failures, show Department of the Prime Minister and Cabinet deputy secretary Allan McKinnon complaining AGs was "ducking for cover" and demanding the department correct the record over the incorrect answers.
When the PM&C deputy secretary finally saw the missing letter, written by Monis to Attorney General George Brandis asking if it was legal to correspond with the Islamic State group, he found it "absolutely unremarkable" in the context of other letters written by the gunman.
Prime Minister Tony Abbott has said confusion over the Monis correspondence had played no role in the siege, which occurred December 15-16 and resulted in the deaths of the gunman and two innocent bystanders, Katrina Dawson and Tori Johnson.
Mr McKinnon's analysis of the letter, which informed his boss's public reaction, was that it would not have made much difference to the review team even if it had been handed over.
"It is like so many other Monis letters, a bit strange, attempting to be provocative and among the thousands of other pieces of information, absolutely unremarkable," Mr McKinnon wrote on June 2.
But a day earlier, he was not attempting to hide his frustration with the Attorney-General's Department bureaucrats as the political heat on the government over the missing letter was turned up by the opposition.
"I have rung and spoken to AGD again," Mr McKinnon wrote in an email.
"They are ducking for cover."
The Attorney-General's Department failed to hand over five pieces of correspondence from Monis to the team conducting the review in the wake of the siege but the review team already had four of the letters from other sources.
Labor used the new material to continue its pursuit of Attorney General George Brandis over the affair with shadow attorney general Mark Dreyfus saying the government knew for days it had provided misleading answers before it corrected the record.
"It is now clear that by 1 June, the government knew beyond doubt that it had misled the Parliament, and that the Attorney-General's Department had been directed to correct the record by the Prime Minister's Department," the shadow minister said.
Mr Brandis' office dismissed the criticism.
On Monday, AGs secretary Chris Moraitis told the Legal and Constitutional Affairs Committee that he thought "ducking for cover" was not the most accurate way to reflect his department's conduct.
"Were we running around looking for answers? Yes," Mr Moraitis told the committee.
"Were we running around like headless chooks, if you might use that phrase, colloquialism? Perhaps.
"But ducking for cover is not the language I would have used."