Dirty Politics author Nicky Hager mishandled the information he had, NZ First Leader ' />

Opposition parties blew an "unloseable" election partly because Dirty Politics author Nicky Hager mishandled the information he based his book on, NZ First Leader Winston Peters says.

Speaking at a Post-Election Conference at Parliament today, Mr Peters said Hager's book detailing allegations of dirty tricks campaign run out of Prime Minister John Key's office, was "a strategic disaster".

He said it "made no sense" to release all the information at once in the book, "giving your opponents days and weeks to prepare their defence".

"Everybody knows you drip feed it and as you do that your opponents are constantly having to react, re-react, re-re-react and you keep the pressure on them.

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"Whatever Mr Nicky Hager thought he was doing, he should have taken some sound advice because if he had taken that advice this election would have been dramatically different.

"Instead of people getting sick of Dirty Politics, they'd have been hungering every day's new disclosure. That's how it's done in politics."

That was the first of five reasons Mr Peters offered as to why Opposition parties lost the election which should have been "unloseable".

The second was that a Labour-Greens Government was "never a mathematical possiblity".

The only way Labour and the Greens had a chance of forming a government was with NZ First, "and no one had talked to us".

Whether Labour and the Greens' failure to recognise their position was "arrogance or ignorance, it didn't really matter because the net effect helped National".

Mr Peters said National benefited from the anti-Green paranoia that already existed and was exacerbated by the Greens' demands to Labour, including the finance portfolio in a new government, two deputy prime ministers and an independent audit of Labour's alternative budget.

To sheet it home, the Greens would not rule out having talks with National after the election, Mr Peters said.

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The third reason was that Labour failed to learn from the 2011 election that voters did not like its policies of introducing a new capital gains tax and raising the age of superannuation eligibility to 67.

"Frankly there is an old rule in politics - you don't announce a tax before an election."

Mr Peters said voters did not hear Labour's explanations that those policies would not apply for some years.

"That's how people are, they're busy and they take impressions as much as they take facts."

The fourth factor was "the Internet Party's toxicity with respect to Mana".

Mr Peters said the Internet Party's "Moment of Truth" event was the "final straw" thatsent soft National supporters "back home" to vote for Mr Key.

The final reason was NZ First's lack of financial resources which hampered its campaign. Despite that, NZ First ran the most cost effective campaign, spending about $1.05 for every vote it secured.