The NSW government is in crisis after Premier Barry O'Farrell announced his resignation following revelations he misled a corruption inquiry.

In Victoria, it has prompted a quickfire reform of the powers of the state's anti-corruption body; in Canberra it has put the heat on Prime Minister Tony Abbott, especially over his support of Senator Arthur Sinodinos, who has been embroiled in the same corruption hearing.

Within hours of the announcement on Wednesday, a sensational exchange occurred at the Independent Commission Against Corruption when former NSW energy minister Chris Hartcher's name was raised in relation to information that led to Mr O'Farrell's demise.

Counsel assisting the commission, Geoffrey Watson, asked controversial Liberal identity Nick Di Girolamo if he had leaked information about a $3000 bottle of wine he gave to Mr O'Farrell soon after he won the 2011 election.

Mr Watson asked Mr Di Girolamo how the media learned of the gift in early March even before ICAC investigators.

''I don't believe I told anyone that I'd bought the bottle of Grange, other than [my wife] Jodie,'' he replied.

''Did you tell another politician, a fellow called Hartcher?'' Mr Watson said.

''I don't believe so,'' Mr Di Girolamo replied, adding that he did not leak the information, including to journalists.

Mr Hartcher resigned from cabinet in December after his office was raided by ICAC investigators in relation to a separate inquiry into allegations he and others solicited illegal political donations in return for favours.

In March 2012, Mr O'Farrell learned of allegations that led to the inquiry into Mr Hartcher. He informed the director-general of his department, who referred them to the ICAC. Mr Hartcher did not respond to a request for comment.

The Premier shocked colleagues with his announcement on Wednesday, minutes before the ICAC was presented with a note by the Premier contradicting his claim he never received the $3000 bottle of Penfolds Grange from Mr Di Girolamo.

A day earlier, Mr O'Farrell had told ICAC under oath he did not receive the wine couriered to his Roseville home by Mr Di Girolamo shortly after his March 2011 election victory. This was why he had not declared it on his register of pecuniary interests as required.

But in an extraordinary turn of events, ICAC uncovered a hand-written note from Mr O'Farrell to Mr Di Girolamo thanking him for the gift as well as confirmation the courier company had delivered the wine.

In the witness box on Wednesday, Mr O'Farrell continued to state he has no recollection of receiving the wine, despite the note, delivery confirmation and Mr Di Girolamo's evidence that the new Premier had thanked him for it soon after it was sent.

''I look at this note, I read the note, I accept that it is in my handwriting and I still have no recollection about the receipt of the gift or the bottle of wine,'' he told the commission.

ICAC has heard Mr Di Girolamo, then chief executive of infrastructure firm Australian Water Holdings, had been lobbying Mr O'Farrell to secure a lucrative public private partnership for the company. The firm, which has links to former Labor powerbroker Eddie Obeid, was unsuccessful.

Counsel assisting the commission, Geoffrey Watson, agreed there was no suggestion Mr O'Farrell had behaved corruptly or gave an advantage to Australian Water Holdings.

The Liberals will hold a partyroom meeting next week, after the Easter break, to elect Mr O'Farrell's replacement as party leader and Premier.

The ICAC inquiry into the infrastructure firm of which Mr Di Girolamo was chief executive, Australian Water Holdings, had its final day of public hearings on Wednesday, but the government faces being embroiled in another investigation in less than a fortnight. On April 28, the commission will launch public hearings into allegations against former energy minister Mr Hartcher and his fellow Central Coast MPs Chris Spence and Darren Webber.

The inquiry will examine whether the trio ''corruptly solicited, received and concealed payments from various sources in return for certain Members of Parliament favouring the interests of those responsible for the payments''.

Liberal Senator Arthur Sinodinos, who stood aside as federal assistant Treasurer after giving evidence at the current inquiry, is also expected to be called.

It is unclear if Mr O'Farrell intends to quit Parliament when he formally resigns as Premier next week or remain on the backbench until the election.