Police did not investigate whether the father of murder-accused David Bain had a motive for killing his family, the High Court in Christchurch has been told.
Mr Bain is being retried on charges of murdering five members of his family in Dunedin after an order by the Privy Council in 2007.
The Crown says Mr Bain, 36, murdered his parents Robin and Margaret and siblings Arawa, Laniet and Stephen at the family's home in June 1994.
The defence says Robin Bain killed the family present in the house before shooting himself.
The Crown is continuing to call witnesses in what is expected to be a three-month trial. On Tuesday, the officer second in charge of the original inquiry gave evidence.
James Doyle, a detective senior sergeant at the time of the killings, told the court he believed there was no motive for Robin Bain to commit murder.
But defence lawyer Michael Reed, QC, said police should have investigated an allegation of incest between Robin Bain and his daughter Laniet.
Mr Reed said Mr Doyle did not investigate possible motives for the defence's murder-suicide theory.
However, Mr Doyle told the court there would have had to have been evidence linked with the motive to tie Robin Bain to the crime.
The court was also told of evidence from an acquaintance of Laniet Bain that she was going to reveal the incest the weekend before the killings.
Mr Reed said Robin Bain's depression and the fact that Laniet was working as a prostitute were also factors.
Blood stains on father not tested
The defence questioned Mr Doyle over why blood stains found on Robin Bain's hands were destroyed before they could be tested.
Mr Doyle admitted he had ordered the destruction of blood smears from Robin Bain's hands, along with skin samples and fingernail scratchings after the initial trial in 1995.
Mr Reed said if the blood had been tested with modern DNA tests and been found to be that of David Bain's siblings Stephen or Laniet, it could have exonerated his client and connected Robin Bain to the killings.
"The position today is that we will never know - and and can never know - whether that was Stephen's blood or Laniet's blood."
Mr Reed also questioned the handling of the crime scene, saying too many people had gone through it. Tests were not done in time to see if Robin Bain had gunshot residue on his hand, the court was told.
Mr Doyle was cross-examined about the destruction of other items pertaining to the Bain case and asked to go through correspondence he and Mr Bain's lawyers had in the mid-1990s about retaining items of significance.
Mr Reed asked Mr Doyle why some items were destroyed, even when there was explicit requests to keep them.
Mr Doyle told the court he regarded seriously instructions to keep items pertaining to Mr Bain's case, even though he thought the court action was completed.
However, Mr Reed put it to Mr Doyle that, regardless of that correspondence, various items were destroyed, which Mr Doyle later acknowledged.