Christian Porter has asked the high court to refuse a bid to destroy material collected from the raid on News Corp journalist Annika Smethurst’s home, arguing it “may well be important” for a potential future prosecution.

While the attorney general has said he is “seriously disinclined” to approve prosecution of journalists who have “done no more than pursue public interest journalism”, Porter has intervened to support the Australian federal police in Smethurst’s high court case by asking the court to uphold the validity of the raid warrant and secrecy laws.

In September Porter issued a new direction requiring his consent for prosecution for a range of secrecy offences. Police have indicated the raids on Smethurst and the ABC headquarters were investigating both journalists and their sources and have never ruled out referring the cases for possible prosecution of journalists.

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In their joint submission, the solicitor general, Stephen Donaghue, noted the AFP is still considering whether to give a brief of evidence to the commonwealth director of public prosecutions so the court should “leave it to the trial judge in any future criminal prosecution to determine whether that material will be admitted” rather than order its destruction.

In June the AFP raided Smethurst’s home, investigating an alleged breach of section 79(3) of the Crimes Act, which prohibits “communicating or allowing someone to have access to an official secret without authorisation”.

In April 2018 Smethurst had reported that the heads of the defence and home affairs ministries had discussed draconian new powers to allow the Australian Signals Directorate to spy on Australian citizens for the first time.

Smethurst has argued the offence breaches the implied freedom of political communication because it is not limited to disclosures that are inherently harmful and gives the government “unconstrained discretion” to protect its information even where it is “wholly innocuous, or merely embarrassing” to the government.

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The AFP and attorney general’s joint submission noted the case concerns publication of part of a document marked to indicate it contained “secret AUSTEO” (for Australian eyes only) information, and “top secret” information that had been distributed through communications intelligence channels only.

These markings meant the document required the “highest degree of protection”, because disclosure could cause “exceptionally grave damage to the national interest”, it said.

It argued the defence security manual stipulates documents should not be marked secret merely to “prevent embarrassment to an individual, organisation or agency”.

The AFP and Porter argued the “prescribed document” was obtained by Smethurst contrary to Part VII of the Act and so section 79(3) “restricted the further disclosure of that document by Ms Smethurst if she both intended that further disclosure and she knew that the disclosure of the information to her was contrary to a duty to keep that information secret”.

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Their submission argued the section had a “legitimate purpose” to “reduce the risk of prejudice to the security or defence of the commonwealth from the disclosure of secret information”.

That purpose was “centrally important to the preservation and protection of the system of representative and responsible government”, and was of such importance that it “may justify very large incursions on the freedom”.

The submission argued the law is both necessary and adequate in its balance, rejecting Smethurst’s contention that secrets could be protected with less restrictive laws.

Even if the court sides with Smethurst on the legality of the offence and the raid, the joint submission argued the material “might still be admissible in the event that criminal proceedings are commenced in relation to the facts underlying this proceeding” – including against Smethurst’s source.

That’s because the Evidence Act gives courts discretion to allow the admission of unlawfully obtained material. The submission argued there was a “strong argument” to do so because any breach was not “deliberate or contumelious” because the AFP believed they were authorised.

The Australian Human Rights Commission has also asked the court to appear, arguing it is part of its role to protect freedom of expression and the right to take part in the conduct of public affairs.

The AHRC argued that while the commonwealth has a head of power to to designate prescribed information – and hence prohibit publication by its employees and contractors – that does not extend to people, such as journalists, who do not otherwise have a duty to treat it as secret.