Key Points:

On February 18, 1957, at 6.30pm, Wanganui farmer Walter James Bolton climbed the steps to the Auckland Prison gallows and was hanged for the murder of his wife.

Bolton was the last person to receive the death penalty in New Zealand and now new evidence has emerged that suggests the Crown may have got it wrong.

Bolton, 68, who was known as Jim, was found guilty of murdering his wife of 43 years, Beatrice Bolton, by poisoning her with arsenic.

In the case, the prosecution alleged Bolton killed Beatrice because he was in love with another woman - his sister-in-law Florence Doughty, with whom he had a sexual affair. Lawyers for the Crown claimed Bolton had concocted a potion of arsenic from sheep dip and laced his wife's tea with it on several occasions, requiring hospital treatment, before a large dose killed her on July 11, 1956.

Bolton was found guilty by an all-male jury in his hometown of Wanganui, and lost a Court of Appeal case, despite his claims of innocence.

But was there enough reasonable doubt in the case for our last state-ordered death to be considered an unjustified murder?

New evidence shows that Bolton made statements to police at the time - which were not shared with the jury - admitting he suffered from erectile dysfunction. This affected the relationship he had with Doughty. He also recounted how Doughty seduced him on at least one occasion.

A father of six, Bolton paid large sums of money for his wife's healthcare (he even placed her in a private hospital); and was the only member of the family to agree to an autopsy, which then revealed Beatrice's organs were riddled with poison.

There is also the question of why Bolton persisted with a method of murder that was not working over a period of time - experts agree arsenic does not accumulate in the organs of the body - and 50 years later, scientists still disagree on whether Boton would have had the knowledge of chemistry to make the poison.

Would Bolton have been found guilty of murder before a court today?

That is the question being posed by documentary maker Bryan Bruce, who reveals evidence that was never heard by the original jury on the Bolton case, in the final episode of the television series The Investigator.

Bruce argues that the outcome might have been very different if the jury had heard all the evidence.

"A lot of the case [in court] depended on seeing Bolton as a sexual predator, which he didn't seem to be capable of being. You could have argued a lot more vociferously for reasonable doubt, if you were defending him now," Bruce says. "I think the Crown could equally have argued that Doughty had opportunity and motive to kill her sister."

However, this was not suggested, and a Court of Appeal case claiming the jury could not have found him guilty based on the evidence to hand, also failed. He was hanged less than 13 weeks after being sentenced. His was New Zealand's last execution but it would be another 32 years before the penalty was officially removed from the books of law.

Ultimately, Bruce argues Bolton may have been the victim of small-town judgement, rather than having been convicted on the evidence to hand. "He was probably convicted as much for his sexual morals as for whether he had killed his wife or not... I think that Jim Bolton deserved the benefit of the doubt."

It is a case that demonstrates the dangers of the death penalty.

Bruce says: "From time to time, when someone commits a heinous crime in this country, kills a child perhaps, you hear people saying bring back the death pen- alty... But the law can get it wrong, and death is final."

Fifty-three men and one woman were executed in New Zealand between 1842 and 1957. The death penalty was abolished in 1941, but reinstated in 1950.

The issue of state executions has been in the news again, with Prime Minister Helen Clark announcing last week New Zealand would support a UN initiative to abolish the death penalty worldwide, saying "capital punishment is the ultimate form of cruel, inhuman and degrading treatment".

* The Investigator screens on TV One, Wednesday, 9.30pm