The handling of a letter from Sydney siege gunman Man Haron Monis to Attorney-General George Brandis will be investigated by a Senate committee.
The inquiry will look at what changes were made to the way correspondence to government is handled after the national terror alert level was raised to high in September.
It will also look at why the letter was not passed on to an internal inquiry into the Sydney siege and how incorrect evidence about this was given to a Senate estimates hearing.
Labor won Australian Greens and crossbench support to allow the legal and constitutional affairs references committee to look into the matter.
The committee is to report back on June 25.
The Attorney-General's Department told an estimates hearing this month a letter received from Monis on October 7, 2014 had been referred to a top-level joint review of the siege.
But three days later, Foreign Minister Julie Bishop revealed the letter had not been handed over because of an "administrative error".
The letters have now been passed to officials in the prime minister's department and the NSW premier's office.
The government believes the inquiry is inappropriate because the letter and relating evidence are already before a legislation committee.
Senator Brandis says that committee can still deal with the matter because of a spill-over day following estimates and has indicated that it will.
Monis and two hostages died in the Lindt Cafe in Sydney's Martin Place after a 17-hour siege.
Shadow Attorney-General Mark Dreyfus said the government had failed to provide an adequate explanation as to why the letter did not raise a red flag.
He believes it is the first time all eight crossbench senators outside of the Australian Greens have voted together, reflecting how serious the matter is.
"This is an inquiry the Abbott government did not want to have but the Australian public deserve a better explanation about this matter of national security," Mr Dreyfus said in a statement.
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