Cousin Michael Mayson said he was shocked when a police officer, whom he could not identify, remarked after the funerals for the five slain family members: "David is the enemy and we are going to get him."
He later passed on the information to David Bain supporter Joe Karam.
Mr Mayson was giving evidence on day 44 of the trial of David Bain on charges of murdering his father Robin, mother Margaret, sisters Laniet and Arawa and younger brother Stephen at their Every St, Dunedin, home in June 1994.
The defence claims that Robin Bain killed his wife and three of his four children and then turned the gun on himself.
Mr Mayson said he had seen Robin Bain earlier in 1994 and thought he must have had a terminal condition as he looked a sick man.
A man, who has name suppression, told the court he had owned a shop over the road from Laniet Bain's flat.
Laniet visited it regularly, and had an account there.
The man said one day she came in crying, distressed and upset. She told him that she was having an affair with her father.
He said Robin Bain would come in and purchase things at the weekend when he was visiting her, and the Sunday before the killings he came in and asked to clear Laniet's account in full.
Earlier in the day, a Dunedin prostitute said Laniet Bain told her about being sexually abused by her father in Papua New Guinea and that Robin Bain was the father of the baby she had.
The baby was white.
Earlier witnesses have given evidence of Laniet talking about having a black baby in PNG.
The woman, whose identity is suppressed, told the High Court where 37-year-old David Bain is on trial for the murders of his family, that Laniet worked for her for a few months in 1993.
And she had seen a photograph of a white baby in Laniet's room.
She believed Laniet when she told her about having a baby because she had seen stretch marks on Laniet's body when the two of them were working together with clients.
''I told her to go to the police but she said they wouldn't believe her because she was a hooker,'' the woman said.
And she also said Laniet had told her she went in to the sex industry because of being abused by her father.
Under cross-examination by Crown counsel Kieran Raftery the witness agreed she said in an affidavit in 1998 that when she asked Laniet who was the baby's father, Laniet would not talk about it.
But Laniet had told her about being raped by her father in PNG, the witness said.
