"Once we start saying private correspondence is public property we go down a very slippery slope."
He challenged the Dominion Post publisher Fairfax to confirm he did not leak the report to Ms Vance, but Fairfax executive Paul Thompson would only say "we don't comment on our confidential sources".
Mr Dunne, who has traded on his "common sense" approach to politics, said he'd resigned because "some of my actions after I received an advance copy of the report were extremely unwise and lacked the judgment reasonably expected of a minister in such circumstances".
He said his lapse of judgment was that he canvassed the possibility of leaking the report with Ms Vance but again said he had not gone through with it.
But Mr Henry's report shows he arranged a lunchtime meeting with Ms Vance the day before her report was published. Although Mr Dunne left his office to meet up with Ms Vance, he said he was waylaid and the meeting never took place.
Mr Henry's report also shows Mr Dunne exchanged 12 emails with Ms Vance the day before he was to meet her, but also 49 others between March 30 and April 7, a period when he was on an overseas holiday with his wife.
"I cannot rationally explain why things happened the way they did," he said yesterday.
"I have acted extraordinarily unwisely, even stupidly, and I am now resigned to paying the price for that."
The Prime Minister said he was "very shocked" by the Henry report, accepted Mr Dunne's resignation after telling him he had to release the emails in full or go.
"He has told me categorically he did not leak the report. I want to believe him but the problem is unfortunately the inquiry doesn't rule him out and I can't dismiss the possibility he has because of the information contained in the (Henry) report," Mr Key said.
He said what details had been released by Mr Dunne "of themselves would be inappropriate ... in so much that it discussed GCSB and that was a sensitive matter which I don't think he should have been discussing anyway".
Asked whether Mr Dunne should resign as an MP, Mr Key said that was a matter for Mr Dunne himself but the United Future leader had indicated he would stay on for the current term.
Mr Dunne said he has not yet decided whether he will contest the Ohariu seat he has held since 1984 in next year's election.
Opposition parties were last night calling for his resignation from Parliament with NZ First Leader Winston Peters saying he had filed a complaint with police. Mr Peters said the Kitteridge report was a classified document at the time it was leaked and its premature disclosure was a criminal matter.
Down the years
1984: Elected Labour MP for Ohariu.
1994: Left Labour after Rogernomes quit. Formed Future NZ, and a year later helped set up United Party.
1999: Held seat when National failed to field candidate.
2002: Formed United Future. Got eight MPs in after his "commonsense" approach won over the "worm" in a TV3 debate. Went down to three MPs in 2005.
2005: Had "cup of tea" with then National leader Don Brash. Attacked Greens as "weird people who promote psychoactive drugs and yet are horrified by a GE spud".
2008: Made Minister of Revenue. Backed National, which needed his vote when it does not have Maori Party support. Was the casting vote in law change to Mondayise Anzac Day and Waitangi Day.
2011: Released video mocking his hairstyle. His wife Jennifer Mackrell celebrated his re-election.
2013: Party deregistered as it did not have 500 members.
