David Bain's sister Laniet was planning to "blow the whistle" on incest with her father, and her career as a prostitute the weekend before the deaths of her family, the High Court at Christchurch has heard.

A woman, whose name is suppressed, told the Bain murder trial she spoke to Laniet four days before she and four other members of her family were shot dead in their Dunedin home on June 20, 1994.

Laniet, 18, told her she was going to her family home the weekend before the killings to reveal an incestuous relationship with her father, Robin, and her work as a prostitute.

Laniet had previously been blackmailed by a man over the knowledge of her work as a prostitute, and forced to have sex with him and do "horrible and graphic things", the woman said.

She told the court they had been friends and lived together at a Dunedin boarding house in late 1993, and Laniet had told her about being raped by a family friend in Papua New Guinea at 10 years old, and having a white baby as a result.

She said she saw Laniet naked once when living together, and noticed she had stretch marks down her stomach.

Previous witnesses have told the court of Laniet's claims that she had had a "black" baby and an abortion.

Laniet told her she was working in the sex industry because her parents would not sign the form to allow her to get a benefit.

Today's witness told the court Laniet later quit being a prostitute and began working at a cafe.

The woman said she was interviewed by police, but was told by them she was not going to be a witness for the prosecution.

Questioned by prosecutor Cameron Mander, the woman agreed David Bain also wanted to see the whole family the weekend before the murders, but did not expand on why this was.

She agreed Laniet was a heavy cannabis user and craved affection.

The woman said she was surprised to find that Laniet had lived for a period with her father at a school house at Taieri Mouth, because she did not like being around her father.

David Bain, 37, is on trial for the murder of his parents and three siblings in their Dunedin home on June 20, 1994.

His defence team argue his father Robin, 58, shot dead the family before turning the rifle on himself.

Earlier today, the court heard from a witness who said Laniet wanted to move back to her family home from where she was living with her father because of what he was doing to her.

British Army doctor Sean Clarke, via video link from Birmingham, told the court today he wrote a note to himself to find out what was occurring between Laniet Bain and her father, Robin, because of what Laniet had told him.

Previous witnesses have told the Bain murder trial of Laniet speaking of sexual abuse and incest involving her father.

Mr Clarke said he studied at Otago University and met David Bain about 1991 through a group called Opera Live. The pair became friends.

Bain was happy, cheerful, sporty, had a good sense of humour and was fun to be with, Mr Clarke told the court.

Mr Clarke said he also met Laniet and got to know her through opera productions.

He had arranged to meet Bain on May 27, 1994, at the Otago student union but Bain did not turn up.

Instead, Mr Clarke saw Laniet, who was looking for her brother.

Laniet was quite agitated and distressed, and kept looking at her watch and asking when Bain would get there, Mr Clarke told the court.

He said he spoke to Laniet for about 20 minutes. During general discussion, she suddenly got very upset.

She said she wanted Bain to talk to their mother about her moving back home from a school house at Taieri Mouth, where she was living with her father, because she couldn't stand what "he" was doing to her anymore.

"We had just been talking about her father, so I assumed it was her father," Mr Clarke said.