When Darren Trinder posted the following comment on Twitter, "ICAC - Eddie rorts millions goes free. O'Farrell receives wine and forced to resign. What a waste of public money," he captured an understandable, but mistaken, view.

Barry O'Farrell's resignation has got nothing to do with whether he accepted a bottle of wine, or whether he declared it. ICAC has also got nothing to do with his downfall. O'Farrell was caught out and he has no one to blame for this but himself.

The crucial question is: why did he hang his premiership on the receipt of a bottle of wine?

It's important to go back to see how this unravelled.

When the commission's forensic accountants found a book entry in the Australian Water Holdings accounts that showed an American Express payment of $2978 with a reconciliation that read "Gift to Barry O'Farrell and wife", they had an obligation to investigate.

If it was discovered they had had this information and ignored it, there would be a public outcry. After all, at the time the $3000 wine was being delivered to O'Farrell's home, April 20, 2011, the sender, Nick Di Girolamo, was trying to get the newly elected Liberal government to grant his water infrastructure company a billion-dollar public-private partnership.

A month after the wine was sent, O'Farrell went over the head of his then water minister Greg Pearce and granted an audience to Di Girolamo. When he gave evidence before ICAC Pearce spoke of his annoyance at being summonsed to this "cosy" meeting.

On Tuesday morning the issue of the $2978 entry was raised when Di Girolamo, a generous donor to the party, was in the witness box. He volunteered it was a single bottle of wine and a 1959 vintage, being the year of O'Farrell's birth.

This was the first time anyone at ICAC knew the gift was just one bottle of wine. Investigators had traced the receipt to Vintage Cellars but had no joy in the details of the purchase. The shock at learning it was for a single bottle was evident in the response of counsel assisting the inquiry, Geoffrey Watson, SC. ''Mr Di Girolamo, I mean friends are friends. That's a three thousand dollar bottle of wine,'' he said incredulously.

When asked what he hoped to secure by such a gift, Di Girolamo replied: ''My sincere congratulations on finally getting into office after 16 long, hard winters in opposition.''

Did he ever get a card or a call thanking him? ''Yes, I did. I thought it was a call,'' Di Girolamo said.

It was after the lunch break that O'Farrell was called. He repeatedly and vehemently denied he had received such a gift.

''I'm certain that I would remember receiving a bottle of Penfold's Grange, particularly one that was of my birth year,'' he said. ''I have no idea how much the current vintage Grange would cost but I would understand that a vintage dated the 1950s would require me to declare it.''

This was not the first time the issue of the Grange had been raised. Six weeks earlier, News Corp's Andrew Clennell had sent him a text about whether he had received such a bottle from Di Girolamo. O'Farrell denied receiving the Grange Hermitage. Clennell took him at his word and did not run the story.

After leaving the witness box, O'Farrell chose to repeat his denials before a media conference providing journalists with all the grabs they needed of him staking his premiership on the bottle of wine. These were O'Farrell's words. No one from ICAC had tricked him into a corner. It was O'Farrell who did this all on his own.

The following morning, when O'Farrell's handwritten note thanking Di Girolamo for the ''wonderful wine'' and noting that ''1959 was a good year'' was handed to the commission, it was all over.

This note proved beyond a shadow of a doubt that O'Farrell had misled the commission, the penalty for which is a potential five-year jail term. His position as premier had become untenable and he knew it.

This outcome cannot be sheeted back to the commission. Indeed, no one could have been more glum about this turn of events than the staff at ICAC, where O'Farrell was seen as a loyal and valuable supporter of the institution.

If ICAC had not tendered the note at 10am on Wednesday, Di Girolamo's legal team would have gone into meltdown. They way they saw it, this note went to Di Girolamo's credit.

No one could have foreseen that O'Farrell's intransigence both in and out of the witness box would have such a dire outcome. But trying to blame the corruption watchdog for this is misplaced.

ICAC has well and truly done its job in exposing serious political corruption in this state involving the actions of the corrupt former powerbroker Eddie Obeid, his family and others, including some of the richest and most powerful businessmen in the state. It has done so without fear or favour. Captains and kings have been treated alike and the fact that O'Farrell became an unwitting victim is no one's fault but his.

A final report on this inquiry and previous corruption inquiries, which have embroiled the Obeids, will be handed down later in the year.

Decisions on whether criminal charges flow are entirely in the hands of the Director of Public Prosecutions.

Eddie Obeid's corrupt behaviour over a $30 million windfall his family received for a corrupt coal deal is already being looked at by this office.