Police on June 15, 2017 bugged a meeting room at Risdon Prison north of Hobart as part of their probe into Neill-Fraser's former lawyer Jeffrey Ian Thompson.
Mr Thompson, who was involved at the time in Neill-Fraser's ultimately unsuccessful court appeal, was being investigated over suspicions he perverted the course of justice in relation to a potential appeal witness.
He was charged but the case against Mr Thompson was abandoned earlier this month.
In recently published judgments, Justice Michael Brett ruled the recording of Mr Thompson speaking with the potential witness was obtained by police in contravention of Australian law and couldn't be admitted.
He also ruled the warrant obtained by police to set up the recording device was invalid, as it didn't properly specify the crime by reference to the correct section of the criminal code.
Justice Brett found the devices were recording for more than two months in the room where sensitive and privileged conversations between lawyers and their clients occurred.
The Australian Lawyers Alliance and Tasmanian Prisoners Legal Service have called for an inquiry into the matter.
Greg Barns SC, who represents both groups, said he was deeply concerned by the revelations.
"This is a fundamental breach of the rule of law. Individuals who meet with their lawyers are entitled to know they're not being listened to," he told ABC radio.
Justice Brett said it appeared police didn't deliberately set out to break the law, but there was an "obvious misunderstanding or ignorance" of significant risks.
"Because of the importance of this question of privacy in the circumstances of this case, I regard this as a significant factor telling against the admission of this evidence," he said.
"I have no difficulty inferring ... there would have been many sensitive and privileged conversations between lawyers and their clients ... which were completely irrelevant to this investigation and not authorised for recording by the warrant.
"The persons concerned would have had absolutely no idea that their conversations were being recorded and were capable of being monitored, in real time, by police and other authorities."
Justice Brett said the police constables involved said they didn't monitor any other conversations or download any other audio.
However, he noted there were at least five members of the investigative team and a number of technical service unit members who had access to the relevant equipment.
He said the passcode needed to access the monitoring equipment was known by all members of the investigative team.
Senior government minister Roger Jaensch said he was unable to comment because police were investigating.
"The matter has only come to light recently. I won't speculate on what we might do with ... advice before we receive it," he told reporters on Wednesday.
State Greens MP Rosalie Woodruff has called on Police Minister Felix Ellis to provide further information.
"As a consequence of this bungled process, evidence in relation to serious allegations will not be admitted to court. This is a lose-lose situation for justice," she said.
Neill-Fraser, who recently became eligible to apply for parole, is serving 23 years' jail for murdering partner Bob Chappell on the couple's yacht on Australia Day 2009.
Her latest appeal bid was knocked back by the High Court earlier this month.
Tasmania Police has been contacted for comment.
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