They are trying to label it a "cross-party" inquiry. They are even claiming it is an "independent" inquiry. They argue it will be the equivalent of a select committee inquiry.

No matter how Labour, the Greens and Progressive MP Jim Anderton try to dress it up, their informal inquiry into the behaviour of the Australian-owned trading banks has no official standing.

It is a political stunt. But stunts are what Opposition is all about.

This stunt may have a serious purpose. The inquiry's stated task is "to focus on the relationship between the official cash rate (set by the Reserve Bank) and short-term interest rates".

But it is still a stunt. Its real task is to embarrass the Government which blocked Labour's bid to get Parliament's finance and expenditure select committee to hold a formal inquiry.

There is no question that going after the banks is a potentially fruitful strategy for the three Opposition parties, especially as National, after much huffing and puffing about the banks' failure to pass on reductions in the official cash rate has adopted a more softly-softly approach.

But there is a question of whether running an inquiry is tactically astute.

The big risk is that the whole thing turns into a fizzer. If it is simply going to be a passing parade of individuals and organisations listing their gripes with the banks, it will serve little useful purpose.

Given the amount of public debate and reports already produced by the finance and expenditure committee, it is difficult to see this inquiry coming up with much that is fresh by way of findings or recommendations. But the onus will be on it to do so.

Labour's finance spokesman David Cunliffe, who has been leading the charge against the banks, is stressing the inquiry will produce a high-quality report with workable recommendations and be subject to independent international analysis.

The inquiry will stand or fall on two things: whether the Reserve Bank makes a submission and whether the banks show up.

If Reserve Bank Governor Alan Bollard fronts, it would be a spectacular coup for the Opposition, giving the inquiry credibility and thus embarrassment for English - which is why that is unlikely to happen.

Perhaps more crucial is whether the banks turn up. Fighting its own transtasman battle via an advertising campaign, Kiwibank has little to lose in doing so - and probably something to gain publicity-wise. Likewise for other smaller operators like TSB Bank.

As for the Australian-owned banks - ANZ, BNZ, the National Bank and Westpac - they may welcome an opportunity to justify their profits and interest rate margins. But they will feel their role in proceedings has been predetermined - that of financial pariahs about to be fed to the lions.

At the same time - as Jim Anderton argues - failure to front would speak for itself. Either way, the banks cannot ignore the potential public relations damage from appearing or not appearing.

One solution could be for the banks to make a joint written submission rather than be picked off one by one - and leave it at that. But it would be unusual to make a submission and then not speak to it.

If the inquiry is to have real political oomph, however, Labour, the Greens and Anderton need to get the banks to take part. The banks know that. It gives them some leverage in how the inquiry is conducted. Without them on board, the inquiry risks being little more than an exercise in whistling in the wind.