Police will tomorrow examine the original fingerprints taken from Robin Bain's body and test the firearm that was used to kill him and most of his family in 1994.
His son David served 13-1/2 years in prison for the murder of his parents and three siblings before being acquitted in a 2009 retrial. His defence team claims that Robin Bain was the killer.
The say that marks on Robin Bain's thumb, not made public until June this year, are consistent with him having fired the gun and prove David Bain's innocence.
Police have rejected the theory, saying the marks were cuts, but they will nonetheless carry out the tests in Christchurch tomorrow.
"This is being done to establish if greater clarity can be reached to explain the marks on Robin Bain's thumb as raised in the media in June," said Acting Assistant Commissioner (Investigations) Glenn Dunbier.
The tests would be conducted by police fingerprint and firearms experts, along with ESR scientists, and analysis of the results may take weeks to complete.
"Until we have the full results of the analysis our position remains that any explanation for the marks remains a theory," Mr Dunbier said.
"As we've said before it also remains a real possibility that, even after the analysis is completed, there will still be no definitive conclusion regarding the marks."
Advocates for David Bain were invited to attend.
"I'm very confident that the original fingerprints will show in fact that there are no cuts on (Robin) Bain's thumb and forefinger where the black marks were," long-time supporter Joe Karam said last week.
"The only proposition they've (the police) put forward so far is that they're cuts from rose bushes or something. Well, I can tell you now with a great deal of confidence, the fingerprints will not reveal any cut."
The marks apparently went unnoticed for 19 years until TV3's 3rd Degree revealed that a gun expert had re-examined photos of Robin Bain's hands, and said the marks matched those made by gunpowder residue when loading the magazine of a rifle shortly after it had been fired.