The NSW Police Force is seeking to prevent the Australian public from accessing more than half of the evidence being examined as part of the inquest into the Lindt cafe siege, including a tranche of documents that are already in the public domain.

It came as Attorney-General George Brandis told a Senate hearing in Canberra that his office and department had regarded a letter written by Monis to Mr Brandis asking about the Islamic State terror group as "routine".

Senator Brandis said the letter had not contained any indication of support for the group, though it referred to the Islamic State leader as "Caliph", which is the title used by its supporters.

As the inquest into the December 15 siege entered its third day on Wednesday, it also emerged that the NSW Director of Public Prosecutions is seeking to prevent the inquest from examining the question of why Monis was released on bail before the siege, given his criminal history and the serious charges pending against him.

Late on Wednesday afternoon, barrister David Jordan, representing the Office of General Counsel of the NSW Police Force, said that the police were opposing "portions of the brief".

The reason for this, he said, was that the evidence could be "prejudicial to a forthcoming criminal trial".

The media have already been prevented from discussing any of the details of this trial after the NSW State Coroner, Michael Barnes, made a far-reaching non-publication order.

The inquest heard that the list of documents that the police force were seeking to hide from the public included judgments of the High Court and other reports that were already in the public domain, including one by the Australian Communication and Media Authority.

"How can we justify suppressing any report that is available on the internet?" Mr Barnes asked.

Mr Jordan acknowledged that this was a complication.

In relation to hiding documents that were already in the public domain, Counsel Assisting the inquest, Jeremy Gormly said it was "hard to understand the policy that underlies the attempt to have that not released".

Mr Gormly also said that "as to the balance of it [the application], I just opposed it completely".

The application comes after the NSW Director of Public Prosecutions indicated that it would oppose any discussion during the inquest of how and why Monis was released on bail in May last year despite facing more than 40 charges of sexual and indecent assault, and having already been convicted of sending offensive letters to the families of Australian soldiers who died in Afghanistan.

Monis was released six days after the state's bail laws were amended to introduce a presumption in favour of granting bail to most offenders unless they posed an "unacceptable risk" that could not be mitigated by strict bail conditions.

Part of the purpose of the inquest is to examine whether the DPP acted "adequately" when it elected not to seek a revocation of Monis' bail when dozens of sexual and indecent assault charges were laid against him.

The inquest continues.

In the letter that Monis sent to Senator Brandis less than four weeks after ASIO raised the terrorism threat level to "high", the self-styled sheikh asked whether it was legal to write to the head of the Islamic State, Abu Bakr al-Baghdadi.

"I would like to send a letter to Caliph Ibrahim, the leader of the Islamic State, in which making some comments and asking some questions [sic]. Please advise me whether the communication is legal or illegal," he wrote.

Senator Brandis said he did not reply personally, but that it was referred to his department to reply, because "it was classified as routine correspondence given the nature of what Sheik Man Monis
asked".

Senator Brandis told the Senate hearing that the tone of the letter was "not obviously threatening" and did not contain any indications of support or endorsement for the terror group.

However, "Caliph Ibrahim" is the name Islamic State supporters use for Baghdadi.

CLARIFICATION: An earlier version of this story was amended to clarify the reason for NSW Police Force representatives opposing certain evidence being presented to the inquest into the Lindt cafe siege.