There is no political “convention” more misguided and less examined than the supposed rule that legal advice to ministers must remain confidential. This is the basis for the government’s refusal to publish the attorney general’s advice on Brexit, instead releasing a summary. The refusal relied on the theory of attorney-client privilege – namely that counsel’s advice to a client is confidential to that client who has the sole discretion over whether to publish it. Ministers, however, are no ordinary clients.
Ministers expend taxpayers’ money when instructing counsel to provide an opinion on the law – a law that everyone is entitled (and indeed presumed) to know. In so doing they act not for their personal interest but on behalf of the people whose interest they are bound to represent. In any true democracy, the public should be able to see that advice, to discuss and debate it, and since it is not infrequently proved, later in court, to have been wrong, to expose its errors before the government acts unlawfully or mistakenly.
And he's got a point. Fundamentally, its our advice after all: ministers work for us, we have paid for it, so we have a right to see it. Publishing it or making it available under freedom of information laws will mean we will know what Ministers think the law means, whether it accords with public understanding, and whether they have been warned they might be breaking it. It will increase both public understanding and the accountability of ministers and officials. But Robertson goes further: as a lawyer, he knows lawyers will argue whatever the client wants them to, so he suggests that the instructions for legal advice also be published. That way, people can know whether ministers are asking whether an action is legal, or whether it is "arguably" legal (or as John Key would have put it, "pretty legal") - and there is a world of difference between the two. We expect a higher standard from government than mere "arguable" legality, particularly where human rights are concerned.
I don't think this is a good idea in the context of litigation (at least until it is finally resolved), but should apply absolutely in the case of ministers and officials seeking advice on the lawfulness of a particular policy. What harm could arise? They won't stop doing that, especially if its legally required as part of the policy process. The only foreseeable "harm" is to limit the ability of government to play fast and loose with the law. And IMHO that is not a "harm" at all.