With that bumbling, mumbled apology on Tuesday, the Attorney-General's Department's new secretary, Chris Moraitis, may have spared himself and his minister, George Brandis, from a criminal investigation.

How so? In one view of the law, without Moraitis' missing notes, the Australian Federal Police is powerless to act on Labor's urging that it investigate whether Brandis sought to bribe the Human Rights Commission president, Professor Gillian Triggs, into resigning.

Put aside the question of whether the AFP should even contemplate scrutinising the minister it reports to (it would be a very poor look, unless Brandis stood aside). The real barrier to any inquiry into this affair is effectively insurmountable: parliamentary privilege.

It isn't widely known but privilege is about far more than encouraging witnesses to speak freely (or, as it is often characterised, a chance to defame others from the safety of "coward's castle").

Privilege offers all parliamentary witnesses near limitless protection, well beyond the reach of statute law. Odgers' Australian Senate Practice, the bible of parliamentary procedure, says any retaliation or "the infliction of any penalty on a witness in consequence of their giving evidence may be treated as a contempt" of Parliament.

What's that mean for Moraitis and Brandis? The federal police, or any other investigating authority, would be unable to question them about their discussions with Triggs, because that evidence would be useless in a court – section 16 of the Parliamentary Privileges Act makes that clear.

And because the Senate, through question time and its estimates hearings, has already examined the Triggs affair exhaustively, there is almost nothing more that Moraitis or Brandis could say that wouldn't be "tainted" evidence: information already subject to privilege. At least, that is one way of reading this area of law, which is largely untested.

There is one exception: Moraitis' missing notebook, on which he said he recorded notes of his conversation with Brandis about whether, and under what circumstances, Triggs would quit her job.

If found, the notes could be used as evidence, which would help the AFP decide to pursue (or reject) an investigation of whether Brandis or Moraitis offered a "corrupting benefit" to Triggs – an offence that carries a five-year jail term.

Moraitis said he "travelled to three countries in two weeks and I have lost those notes, losing my briefcase by mistake". Ironically, he is also the public servant who oversees Australia's information security laws as well as the management of government records.

I am sure he will do his best to rectify this embarrassing cock-up – but Labor shouldn't hope that his best will be enough.