Yo ho ho and a bottle of wine. Or two bottles of wine. Or three. It seems the only limit is how much MPs have already chewed up of their $14,800 annual expenses allowance.

That is the astonishing upshot of the mind-boggling 20-page report by Auditor-General Lyn Provost which follows her investigation into Phil Heatley's credit card spending while he was Housing and Fisheries Minister and which included the purchase of two bottles of wine at last year's National Party conference.

Heatley's mistake, it seems, was to use his ministerial credit card when he could have justified the purchase under the guise of "parliamentary business" and dipped into his MP's expenses allowance to pay for the wine.

The report effectively gives MPs licence to use the expenses allowance to splash out on wine for their mates at their party's annual conference - in fact, at any party meeting.

Of course, had Heatley done so, no one would have been the wiser. It would have been impossible to tell whether he was using money from his base salary or from the expenses allowance. No one would have asked. And Heatley would not have had to resign from the Cabinet.

In doing so, however, he has shone light on something which is pretty murky. The hefty expenses allowance is paid as a component of MPs' salaries to ensure they are not out of pocket from carrying out official or parliamentary duties. That is fair enough. MPs are constantly hit up for donations to worthy causes and obliged to buy raffle tickets to help fund organisations in their electorates. They should not have to suffer financially for such largesse.

But purchasing bottles of wine for electorate delegates during the Saturday night social at the National Party's annual conference stretches the definition of "parliamentary duties" to a point well beyond public comprehension and tolerance.

Then again, those whose blood pressure soars at the merest mention of MPs' perks would be advised not to read Provost's report for health reasons.

They would be better to wait for her next one. That will see the Auditor-General completing the terms of reference of her inquiry by (hopefully) sluicing out the the Aegean stables which the MPs' expenses mess has become and make some wide-sweeping recommendations for a regime which is open, transparent, fair to the taxpayer and properly administered.

Particularly worrying is yesterday's report's revelations of the slack manner in which parliamentary authorities implemented rule changes which resulted in them continuing to pay some of the Heatley family's travel bills when they should not have been doing so.

The report is gob-smacking, jaw-dropping stuff, not least in its exposure of Heatley's treatment of the Internal Affairs VIP transport service which included getting the service to provide a driver to drive the minister's car from Blenheim to Kaikoura while he and his family went by train.

A further VIP car had to be sent to Kaikoura to get the driver back to Blenheim.

Despite that, however, there is only some mild tut-tutting in the report at Heatley's failure to adopt a "more conservative approach that took greater account of how others might perceive his use of public money".

The report basically clears Heatley by concluding his misuse of his taxpayer-funded ministerial credit card was not deliberate and was in part explained by his not understanding the rules.

That probably left the Prime Minister little option but to reinstate him as a fully-fledged Cabinet minister in the portfolios he held before his resignation last month.

Had he not done so, John Key risked setting an unrealistically high benchmark that would mean every time a minister made a minor mistake or lapse, he or she would have to go.

Had the Auditor-General been more caustic, however, the saga may have had a far less happy ending for Heatley.