It was 9.17am when the death warrant for Premier Barry O'Farrell's political career arrived at the Independent Commission Against Corruption.

Written in neat upright strokes with a blue fountain pen, it was Mr O'Farrell himself who had penned the gracious "thank you" note that would bring an end to his premiership.

The previous day Mr O'Farrell had staked his leadership by categorically denying – under oath – that he had received a $3000 bottle of 1959 Penfold's Grange Hermitage, a month after he became the state's 43rd premier.

The note, which had arrived by email, had been sent by Todd Alexis, SC, who is representing Nick Di Girolamo, the man at the centre of the corruption inquiry.

As soon as he read the note, in which Mr O'Farrell thanked Mr Di Girolamo for the "wonderful wine from 1959", counsel assisting the commission Geoffrey Watson, SC, realised the dreadful implications for Mr O'Farrell. Commissioner Megan Latham told the hearing she was informed at 9.20am.

Commission staff then contacted Mr O'Farrell's barrister, John Agius, SC, who is understood to have told the Premier of the grim news.

Mr O'Farrell decided immediately that his position was untenable. He knew that the note would be made public when the commission commenced at 10am so he hastily called a news conference to start at that time. He quickly sent a text message to Prime Minister Tony Abbott excusing himself from their joint news conference about the second airport at Badgerys Creek.

"Commissioner, just before we resume the evidence, there's been a pretty substantial development on a significant matter overnight," Mr Watson told the inquiry at 10.10am. He then tendered Mr O'Farrell's handwritten note along with the envelope addressed to Mr Di Girolamo. Just as exhibit C115 was being tendered, three blocks away the Premier delivered the shock news that he was resigning.

Less than 24 hours earlier, the inquiry heard that Mr Di Girolamo, who was seeking a billion-dollar public private partnership from the O'Farrell government, had couriered the bottle of Grange to Mr O'Farrell's Roseville home on April 20, 2011. Phone records also revealed that Mr O'Farrell made a 28-second phone call to Mr Di Girolamo that same night.

Despite being shown the courier's invoice and the call records, Mr O'Farrell vehemently denied receiving the wine. "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."

Overnight Mr Di Girolamo found the note and gave it to his barrister.

On Wednesday, Mr Watson moved to quash speculation that the commission had sat on the note in order to trap Mr O'Farrell. "There is some suggestion being made that ICAC had access to that information relating to Mr O'Farrell, the card, and sat on it, held it back until Mr O'Farrell gave his evidence." This was false, he said.

"I can tell whoever wants to know in the world that ICAC acquired the information at 9.17am this morning."

Just after noon, looking tired and drawn, Mr O'Farrell found himself back in the hot seat at the ICAC. "Well, Mr O'Farrell, it's in a pretty sad position that we are now. I've got to ask you, why should the people of NSW, why should they not think that you didn't give honest evidence yesterday, Mr O'Farrell?"

Mr O'Farrell replied: "Well, I certainly tried to give, ah, ah, accurate evidence to the best of my recollection." He said he was "clearly mistaken" and "I certainly regret that".

The Premier also said: "It is a matter that I deeply regret as someone who has always defended this institution. But I say again, Counsel Assisting, I look at this note, it is clearly in my handwriting, it has done nothing, it has done nothing to refresh my memory about the delivery or the, or the, or the destiny of that bottle of wine."