John Banks has finally emerged from a Kafkaesque nightmare after the Court of Appeal acquitted him on a charge of allegedly filing a false electoral return.

For a serving Cabinet minister to have had his political career destroyed as a result of his unfortunate tangle with Kim Dotcom is one thing.

Banks had hoped to finally prove his innocence at a Court of Appeal-ordered retrial after his wife Amanda tracked down two Americans, said at the initial court trial to have been at a luncheon at the Dotcom mansion where two $25,000 donations to Banks' 2010 Auckland mayoralty campaign were allegedly discussed.

Mrs Banks got signed affidavits from them which said donations were not discussed at the lunch.

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But when the Court of Appeal this week found that the Crown had then gone on to mislead the court - this even after Amanda Banks' detective efforts - it had no option but to throw out plans for a retrial and acquit Banks.

It's incomprehensible that it was not until February this year that Banks' defence team was given a memorandum containing Dotcom's claim there had in fact been two lunches. He claimed the Americans were present at the first. But the donations were only discussed at the second.

The Court of Appeal said this was "a serious error of process" on the Crown's part which had led to a miscarriage of justice.

Attorney-General Christopher Finlayson says he is looking into the matter.

So he should.

Clearly public doubts have been passed on the integrity of the Crown Law Office and that of prosecuting lawyer Paul Dacre, QC.

The Court of Appeal would not have ordered the retrial in the first place if it had known of the latter memorandum.

A sitting Cabinet Minister lost his job. There now needs to be an independent investigation into what actually took place after the retrial was ordered.

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It needs to ask why the memorandum was overlooked and more particularly why Dotcom and his witnesses have now come up with a new version of events.

Banks is made of tough stuff. But he has cut a pitiful figure about town since Justice Edwin Wylie found him guilty of filing a false election return in the High Court hearing last June.

He has indeed been lonely. His marriage to his very loyal and decent wife Amanda has been on hold. She went to Queenstown. He stayed in Auckland at their plush apartment in the Stamford Plaza hotel complex.

When people bumped into him outside the hotel as he shuffled about staccato style - fists clenched with his straight arms tucked into his side - Banks would not meet their eyes. He went from being "John" to the many Aucklanders who he had glad-handed in the way politicians do, to being a figure of embarrassment.

Banks' wife did not take kindly to having her honour impugned.

The former pharmacist who has owned her own businesses would have smarted when Justice Wylie preferred the evidence of Dotcom's former wife, Mona, over her own.

It will also stick in Banks' craw that when it came down to it, Justice Wylie preferred the Dotcom camp's recollections.

The outstanding question is whether there was indeed a conspiracy against Banks as the former politician claimed during his initial trial.

When Judge Edwin Wylie entered the initial guilty conviction, I wrote: "Banks is a smart man. He is intensely loyal. He needs to reflect on what has happened. Then do the decent thing and resign."

Banks did finally resign his warrant as a Cabinet minister. He was unable to stand again as a political candidate at the 2014 election.

He deserves compensation.

There are lessons in this affair for politicians.

Banks is one of several former Cabinet ministers whose careers have either come unstuck or had their political reputations besmirched after they tangled with major donors.

There is plenty still about the two Dotcom donations to Banks' campaign for the Auckland Mayoralty in 2010 that remain (at best) simply plain sloppy.

And there's a lesson also for all politicians and party bagmen about the necessity to be absolutely tight on process to ensure that no one - least of all a Crown prosecutor - can drive a truck through that skilful political craft of prising anonymous donations out of the pockets of carefully defined donor targets.

But the prime issue is the behaviour of the prosecutors.

This demands an independent probe.