There is more to the metadata retention controversy than the attorney general’s grasp of his brief. He and the more tech-savvy Malcolm Turnbull had form in opposition. Then, they claimed to regard Labor proposals to much the same effect as unjustified incursions on privacy – but the terrorist threats to Australia have not shifted enough, quantitatively let alone qualitatively, to explain their apparent shift of perception. Perhaps the fact that they were then in opposition and are now in government explains their shift.

An important aspect of the debate is how vehement serious commentators are in their condemnation of the proposal, which would require businesses to retain for two years information that may not be significantly more revealing of private lives than records already kept.

Until we are told by the government what, precisely and intelligibly, is intended to be caught up in the annoying word “metadata”, we cannot know whether to be reassured or alarmed. Nor can we size up the proposed legislative text to check whether it goes so far and no further. The vagueness of the press conference held by the prime minister and the attorney general, combined with the delay in fully briefing the opposition, have not deflected community concerns. It is quite urgent for the promoters of this airy bill to share its details, and explain why it is, in their view, important for national security.

It is remarkable that metadata – whatever the government means by them – can arouse such fierce and sincere protests against their availability to be used by those whose task it is to protect us from terrorist crimes. The period data would be held for is less than the statutory obligations to keep financial records that the tax commissioner can use to audit us. It is also less than the various obligations to keep corporate records for others to rake over. Both those kinds of records could well reveal personal matters, and embarrassing ones at that. As for our medical and banking records, nearly everyone wants them to be retained for future use. Then there are the newer kinds of tracking information we blithely create, potentially against ourselves, such as credit and debit card transactions, as well as travel cards.

Turnbull’s understanding of the extent of metadata seems the best working indication of the proposed changes. The government’s current provisional position simply does not leave a cloak over so-called content. Examples are myriad of the inferences to be drawn from tracking or billing data, without going to specific content.

No one wants to deprive investigators, prosecutors and juries of the private content of the conversations, correspondence, telephone communications or electronic networking of those people who are of legitimate interest to law enforcement. Given that the prevention and eventual punishment of crime must start somewhere, surely we are socially agreed that those people’s otherwise private dealings may be recorded and accessed by our officials. Some may become individuals reasonably suspected of wrongdoing, and some may in turn come to be accused of alleged terrorist offences. The record does not show any disdain in this country for using authentic records of these kinds: our criminal juries routinely hear, see and read such records.

But a data retention law should also suggest its own safeguards, something which has not yet been embraced by its promoters; the government should rapidly start the difficult but necessary task of allaying the fears of civil liberties supporters. Striking the proper balance between some human rights (that is, not to be killed or injured), and others (that is, the rights to privacy, freedom of communication and opinion) is Australia’s duty under the International Covenant on Civil and Political Rights.

The businesses to be bound by the proposed law are reported to be worried by the costs involved, and will probably enlist our hip pocket nerves in favour of that reluctance by asserting their commercial intent to pass all those costs on to us, their customers. But we and they, if they value their goodwill, should be much more interested in the security of retained metadata. That copious information cannot be locked away physically in a safe. Hackers abound, and need not be on a News Ltd payroll to be reckless with the legitimate privacy of others in society.

If those grave concerns can be adequately handled, that leaves what I think is the cardinal point. Without the orthodox protection of a requirement for a warrant from an independent authority, the prospect of massive official access to and use of data will leave an otherwise beneficial law highly problematic. That such deficits in public support for a good tool against terrorism may well proceed from outdated perceptions about our national security and police agencies is no reason to ignore their potent reality.

Insisting on warrants is a small price and no spoiling burden. It brings the scheme in line with present and traditional safeguards applicable to full content data - such as the letter inside the fabled envelope. Details like so-called generic (emphatically not “general”) warrants for certain kinds of enquiries can be sensibly devised. In order to justify this reversal of privacy expectations, the need for warrants should be our guiding principle.