Rarely - if ever - has a Prime Minister been so open in admitting that he, one of his personal staff and departmental officials had got it badly wrong as John Key did yesterday in a fashion which veered close to self-flagellation.

The Prime Minister effectively donned sackcloth and ashes and turned his weekly press conference into a confessional, apologising for the "sloppy" way his Government had dealt with the news that Internal Affairs had gone ahead with buying a replacement fleet of new BMWs to transport Cabinet ministers.

It was an extraordinary display - but one with a purpose behind it.

The fuss over buying replacement cars for models bought little more than three years ago has seriously undermined Key's efforts to build a reputation - and thus a competitive advantage over political opponents - both for austerity in Government and openness of Government .

Questions about what he knew as the minister responsible for the purchase - and when - have also allowed his opponents to raise the matter of his own credibility.

Key argued - correctly - that no other Prime Minister has done more to cut taxpayer-funded perks enjoyed by ministers and MPs.

But he also acknowledged the BMWs sent a dreadful message to voters in terms of the difference between what the Government preached and what it practised.

The irony is that the deal between Internal Affairs and BMW now looks to have been a good one from the point of view of the taxpayer, presuming you believe Cabinet ministers should be chauffeur-driven around in upmarket vehicles to give them some degree of respect and gravitas.

From what Key has said and other information released by the Beehive, the first batch of cars have been sold back to BMW for close to the price they were bought for. Moreover, the deal Labour signed in Government included free maintenance and free new tyres during the contract.

What has aroused Key's ire is that it took all week for his staff to extract this information from stonewalling departmental officials concerned about matters of commercial sensitivity.

After the department refused to let Key see the contract with BMW, the Prime Minister called in Solicitor-General David Collins. Key was of a mind to cancel the contract outright but Collins' interpretation of its contents was that would not be lawful.

Complicating matters was a rare mistake by Wayne Eagleson, Key's chief of staff. The department says he was told about the repurchase last July, but failed to alert Key. Eagleson has said he cannot recall the meeting.

Key insists he found out about the BMW deal only two weeks ago.

But what really lies behind yesterday's mea culpa is preservation of Key's brand, which has made him so popular at least in part because he does not come across as the kind of excuse-filled, prevaricating politician interested only in self-preservation and which the public finds so repellent.

Yesterday's apology was designed to underline that difference. The public must now decide if it will buy it.