In his autobiography Incredible Luck published today, the man who led the National Party for three years says the end of both his marriages had been "hugely painful" - and in both cases had led him "to the point of contemplating suicide".
In a warts-and-all chapter covering his personal life, Dr Brash tackles head-on his reputation for being a womaniser. But beyond arguing that men have an "extremely powerful biological urge to have sex", he struggles to explain why he had extra-marital affairs which ultimately took a huge toll on his personal life and plunged him into a deep trough of depression.
Acknowledging an "occasional" breaking of his marriage vows, the 73-year-old says he has not had a large number of sexual partners - and none at all before his first marriage.
"But I have no choice than to admit that the number exceeds the number of women I was married to."
Dr Brash married his first wife, Erica, in 1964. His marriage broke up after he and his Singaporean secretary, Je Lan Lee, began a relationship.
Following his divorce from Erica Brash, the pair married. His second marriage collapsed following his highly publicised affair with Diane Foreman, the then deputy chairwoman of the Business Roundtable - a relationship which resulted in MPs in the National caucus rounding on their leader over his private life.
Dr Brash rejects as "totally and utterly untrue" rumours that his emails were leaked to political opponents by an unnamed "attractive" member of his parliamentary staff who was angry with him after he ended a relationship with her.
He says he has never had any kind of relationship with that person - or anyone else in the parliamentary complex for that matter.
In an interview with the Herald, Dr Brash said that while he had thought about committing suicide, he had not attempted to do so. He had been "very depressed" after both his marriages ended. "I was at a pretty low point."
As a self-confessed workaholic, he dealt with the depression that followed his first marital break-up by immersing himself in his role as managing director of the Kiwifruit Authority and subsequently as chief executive of the merged Trustbank.
By the time he married his second wife, he was back on "a pretty even keel" mentally.
He said leaving her was the worst decision of his life "because I was then and still am in love with the woman".
He reveals that when he told his mother, who believed marriage should be until death do us part, that he planned to marry for a second time, she swore she never wanted to meet Je Lan.
Dr Brash writes that adultery was certainly not part of his Christian upbringing, but argues "that the great majority of human males are programmed to find women sexually attractive".
He realised, of course, that some men were gay. "I have never suspected, even for a single moment, that I might be gay."
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- John Armstrong of the NZ Herald