A growing number of New Zealanders believe convicted double murderer Scott Watson may be an innocent man.

A new survey, conducted by UMR Research, found that 77 per cent of respondents were either unsure or thought Watson was not guilty.

This continues a trend of growing doubt from polls by the same company in 2002 and 2004 which found 41 per cent and then 56 per cent were unsure of his guilt or thought he was not guilty.

The proportion who now believed Watson is guilty is 23 per cent, compared to 44 per cent in 2004 and 59 per cent in 2002.

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The polls were of "a representative sample of 750 New Zealanders aged 18 or over", UMR said.

Hope, 17, and Smart, 23, went missing in the early hours of New Year's day 1998.

Their bodies have not been found. They were last seen boarding a boat with a lone yachtsman.

The Crown alleged Watson was that yachtsman. He was convicted of murder and sentenced to life imprisonment.

Subsequent appeals failed but the case remains controversial 18 years later because of the absence of compelling physical evidence and the possibility that the pair boarded a ketch rather than the much smaller sloop owned by Watson.

Mike Kalaugher, who in 2001 published The Marlborough Mystery, commissioned the poll.

He suspected the change in attitude was due to published scrutiny of the case, the most recent being the television documentary Doubt: The Scott Watson Case in September.

At the time the documentary screened, a spokesman told the Herald that Police were "unable to publicly re-litigate selective aspects of a historic investigation [but] Mr Watson has the option of raising such matters through the appropriate judicial process".

Researcher Mike Kalaugher at the Auckland High Court. Photo / Jason Oxenham
Researcher Mike Kalaugher at the Auckland High Court. Photo / Jason Oxenham

The poll result showed that older people are more likely to think Watson is not guilty - 43 per cent of retired people, 37 per cent aged 60 plus, and 35 per cent of those aged 45-59.

"I think that the more people find out about the case the less convinced they become of Watson's guilt."

A retired chartered accountant and lifelong sailor, Kalaugher said he was prompted to research his book because the yachting evidence did not make sense to him.

He had hoped his book would lead to the mystery ketch being identified, and said his motivation for commissioning the survey was it was "unfinished business".

The case hung on whether it was Watson who the pair boarded a yacht with.

The Crown said that on the night Watson was looking for a woman to have sex with, and that his alleged movements and behaviour afterwards were consistent with someone who had something to hide.

During the Crown's closing address, the jury was told that the more certain they were that the man on the dingy that ferried revellers, including Hope and Smart, to boats in the bay was Watson, the less concerned they needed to be about the reliability of the description of a ketch.

But Kalaugher believes a mistake was made when it was concluded that Hope and Smart boarded a boat in the same part of the bay as Blade. As police were able to exclude other boats in the area, Watson became "the last man standing".

"I think the police made a very serious mistake when they decided the boat the kids got on to was somewhere near Watson's boat," Kalaugher said. "That was always uncorroborated but it coloured the investigation from then on."

Kalaugher used witness statements to plot where he believes the ketch was in the bay - about 200 metres from Watson's boat, and outside the area covered by photographs produced in court.

He acknowledged that it didn't help that the ketch had never been identified. "We've never got a photo or a name for the boat."

What had happened to the Ketch?

"God knows."

But he said much of the prosecution case had fallen away.

"As of today there are no witnesses who support the proposition that the man boarding a yacht with Ben and Olivia was Watson. There are no witnesses who support the proposition that the boat they boarded was Watson's. There are no witnesses who support the proposition that the boat was in the same part of the bay as Watson's boat."

By The Numbers

GUILTY

2002: 59%

2016: 23%

NOT GUILTY

2002: 15%

2016: 29%

UNSURE

2002: 26%

2016: 48%

Case File:

• 1999: Watson found guilty of murders after 13-week trial.
• 2000: Court of Appeal dismisses appeal.
• 2002: Privy Council rejects leave to appeal.
• 2006: Trial By Trickery, Keith Hunter, critical of methods allegedly used by police to obtain Watson's conviction.
• 2010: Independent Police Conduct Authority report found the police investigation had fallen short of best practice in areas which "had no significant bearing on the outcome of the investigation".
• 2013: Governor-General rejects application for exercise of the royal prerogative of mercy.
• 2015: Parole Board denies parole on the basis of two failed drug tests and an unfavourable psychological report that said Watson was "a very high risk" of committing violent acts if he was released.