The inquiry rocked New Zealand's faith in police after it found that the cartridge case used to convict Thomas was planted by police.
No investigation has been carried out by police into allegations of wrongdoing by officers who investigated the homicides.
Two years ago, Rochelle Crewe, a toddler at the time of the murders, asked Prime Minister John Key for an independent inquiry into her parents' death. In the wake of Mr Key's refusal, the police appointed Detective Superintendent Andy Lovelock to review the file.
The Weekend Herald has learned a rifle has been sent to Britain for testing. It is not known whose rifle it was or why it is being tested.
Mr Lovelock has interviewed some of those connected to the case, including Arthur Thomas' brothers Des and Ray.
Ray Thomas said his interview with Mr Lovelock led to the question of Arthur Thomas' former relationships. "I was disturbed. On my way home I rang Des and said, 'We'd better have a meeting."'
The two brothers met the third, Richard, at his home and discussed the implications. "I felt as though they (police) weren't interested in getting to the bottom of it. Why else would he ask about Arthur's girlfriends?"
One of the women asked about was "Lorna", who was featured in the book Beyond Reasonable Doubt, by David Yallop, and a significant factor in Thomas being granted a pardon.
Arthur Thomas was in a relationship with her about 1960 when working in the Forest Service at Maramarua in north Waikato.
Des Thomas, who last month made criminal complaints alleging wrongdoing by police who investigated the murders in the 1970s, said the questions had been asked 40 years ago when his brother was charged. "We were surprised. It is actually a disgrace."
He had received a letter rejecting his complaint last week. "They shouldn't be able to pick and choose which evidence to investigate just to suit their case."
Arthur Thomas was surprised to hear police were quizzing his brothers over 50-year-old relationships but said he did not want to comment beyond applauding Des Thomas for complaining against police.
"We've just got to get on with our own lives. Honestly, we've had a gutsful."
In an emailed statement, police confirmed interviews had been done "to assist the team in understanding the events which took place in970".
"The review is a thorough analysis and assessment of the Crewe file and the allegations against police in an endeavour to answer questions raised by Rochelle Crewe.
"This does not mean a reopening of the case or reinvestigation."
New inquiry looks at how police acted
Allegations of criminal behaviour by police will be part of the review into the Crewe murders.
The decision comes after Des Thomas, brother of the man wrongly convicted for the killings, made a formal complaint to police asking they investigate.
Assistant Commissioner Malcolm Burgess wrote back this week saying "most" of the issues raised had already been looked at.
But he said the matters raised by Mr Thomas "are issues that fall within the review being undertaken" by Detective Superintendent Andy Lovelock.
Mr Burgess said the correspondence would be given to Mr Lovelock to consider as part of his review.
If he investigates the claims, it will be the first time police have ever probed allegations their officers planted evidence to get a murder conviction.
The royal commission of inquiry in 1980 found police officers planted a shellcase which was then wrongly used to convict Arthur Allan Thomas of killing Harvey and Jeanette Crewe.
The police commissioner and the solicitor-general both ruled the commission was wrong.
The case has been cited as having had a corrosive effect on the relationship between police and the public.
Des Thomas said Mr Burgess' response had failed to appreciate the seriousness with which his family took the actions of the police.
He said the impact of asking Mr Lovelock to investigate instead of holding a separate inquiry would be to distract him from finding the killer of the Crewes.
