Barry O'Farrell, who leads Australia's populous New South Wales state, told the Independent Commission Against Corruption (ICAC) yesterday that he was never given the 1959 bottle of Penfolds Grange Hermitage.
But when advised that a thank you note he had signed would be presented at the inquiry today, O'Farrell quit.
"I've accepted that I have had a massive memory fail," he told reporters in Sydney.
"I still can't explain either the arrival of a gift that I have no recollection of, or its absence, which I certainly still can't explain."
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Di Girolamo said the gift -- a 1959 vintage to mark the year of O'Farrell's birth -- had been couriered to the Liberal Party premier's Sydney home to offer "sincere congratulations" on his election victory.
The inquiry has been investigating how Di Girolamo's company, Australia Water Holdings (AWH), was allegedly able to bill a state-owned firm for luxury travel, million-dollar salaries and major political donations.
It has been alleged that the company stood to make up to Aus USD 60 million if its proposal for a public-private partnership with the government was approved. The deal never went ahead.
He said he would formally resign at a meeting of the New South Wales parliamentary Liberal Party next week, enabling the election of a new state premier.
Prime Minister Tony Abbott, who leads a Liberal/National Party coalition government, said O'Farrell had "inadvertently" misled the corruption watchdog and he maintained enormous respect for the departing premier.
"We are seeing an act of integrity, an act of honour, the like of which we have rarely seen in Australian politics," Abbott said.