Key Points:

Don Brash has resigned as leader of the National Party after weeks of speculation about a coup against him.

Dr Brash, who took over as leader in October 2003 after rolling Bill English in a coup, called a press conference at 1pm after mounting revelations about his leadership in a book by researcher Nicky Hager.

Ongoing speculation about his leadership was damaging the National Party and its election prospects, he said.

His resignation would take effect from a special caucus meeting on Monday, he said.

Don Brash told reporters: "It has become increasingly clear in recent months that there's a growing expectation that I'll step down well ahead of the next election.

"That ongoing speculation is damaging to the National Party, and to our future prospects.

"I've decided to resign as Leader with effect from a special caucus meeting which I'll call for early next week. "

Dr Brash said that he had been considering resigning for several weeks.

"For some weeks now, I've been giving consideration to the right time, and the right way, to announce this decision.

"I've held off because I've been very keen to have two untidy matters dealt with before my departure."

He said that two matters had needed to be cleared away; National's problems with GST during the election campaign and his stolen emails.

On the question of his emails turning up in Mr Hager's book, The Hollow Men, Dr Brash said: "We commissioned a formal investigation to see if we could discover the source of these emails, but found nothing."

"In early November, I decided that the publication of a "book" of emails to and from me would be totally unfair to the many hundreds of people who correspond with me by email, and would potentially hinder my ability to operate as a Member of Parliament and as the Leader of the National Party.

"Great was my surprise when, two days ago, what got caught by the injunction was not a book of my emails, but a scurrilous book by Nicky Hager attacking both the National Party as an institution and me personally."

Dr Brash said he utterly rejected Mr Hager's "conspiracy view of the world" and he claimed the book was an attempt to discredit the National Party and him as leader.

"It's simply nonsense to suggest that the National Party was, or is, under the influence of American neoconservatives; or received funding from the Exclusive Brethren; or broke election spending rules – the only party found to have broken the election spending cap was the Labour Party."

Dr Brash said he would open the way, though, for Mr Hager to publish the book by providing him with his own emails, thus getting around the injunction.

"I anticipate that Mr Hager will be able to publish his book tomorrow morning.

"Unfortunately, that will not protect the identity of people who have sent me emails, and on the assumption that Mr Hager has not obtained their permission to publish their emails, I regard his behaviour as totally reprehensible."

Dr Brash refused to say whether he would continue as an MP, telling reporters if he was offered a spokesperson or senior portfolio role he would be inclined to stay on but if he was offered "deputy assistant spokesman for consumer affairs" he probably wouldn't.

Dr Brash has transformed National into a respected, potent and feared Opposition after it took a record drubbing in the 2002 election.

He came within a sniff of winning the 2005 election, but his leadership has been marked by a series of gaffes that have raised questions about his judgement.

Dr Brash took over as leader in October 2003, rolling Bill English in a coup.

Even that did not run smoothly, with Nick Smith being appointed his deputy, then having to be hastily replaced by Gerry Brownlee.

Dr Brash reignited National as a political force with his controversial Orewa speech in January 2004.

He used race relations to fire National to support in the high 40 per cent range, one of the most dramatic turnarounds in New Zealand political history.

But Dr Brash was to prove fatally flawed as a politician.

Political consultants Saunders Unsworth described him as a true gentleman -- but not good on policy detail.

Dr Brash also proved naive, attacking the Labour Party on areas where he was himself vulnerable, such as the sanctity of marriage when he had himself had admitted having an affair.