Murder accused Ewen Macdonald swore to his wife on one of her prison visits to him that he didn't kill her brother Scott Guy.

Anna Macdonald on Thursday gave evidence in the murder trial of her husband who is accused of shooting his brother-in-law dead on July 8, 2010, in the driveway of the family's Feilding farm.

He has admitted torching the Guys' old farm house in 2008, when it was on a truck ready for moving, and also criminal damage to their new house as it was being built in 2009, but denies the murder.

Mrs Macdonald told the High Court at Wellington on Thursday that she had visited her husband in prison after he was arrested and asked him whether he had killed her brother, according to media reports.

"He said: `no I swear I didn't'."

Macdonald acted differently after the damage to the Guys' house. Although she didn't realise it at the time, he had been trying to make up for what he had done, Mrs Macdonald said.

Macdonald told his wife that he was annoyed at Mr Guy for going home whenever his wife Kylee called him.

Offensive graffiti written on the house had been directed at Mrs Guy, Mrs Macdonald said.

She told the court she could not believe he felt so cross and upset about things and couldn't talk to her.

Earlier the Crown alleged three abusive hand-written notes found in the Guys' letterbox were written by Macdonald and were part of his attempt to drive the Guys off the farm.

Macdonald denies writing the three notes, which were found around the time of the attacks on the two houses.

They said "You cheating whore, what goes around comes around", "Stay away from him Kylee, you whore" and "Now you know how it feels to lose something you love."

Two posties testified that they had found the notes in the letterbox.