West Australian Nicolau Francisco Soares, 28, is alleged to have killed Dr Delys Weston and Professor Gavin Mooney at their Mountain River property in the state's south in December 2012.

Mr Soares pleaded not guilty to murdering the two academics in the Supreme Court of Tasmania in Hobart last week.

The trial commenced today, and the jury has been warned by Justice David Porter that they will have to view images of the bodies of the deceased.

``This can shock, and distress people,'' Justice Porter said.

Crown prosecutor Linda Mason told the court Dr Weston and Mr Mooney were attacked while eating dinner.

 ``You'll be left with no doubt that this is a horrific crime,'' Ms Mason told the court.

Ms Mason told the court Mr Soares had long-standing mental health issues, and it would be up to them to determine if he was so unwell it would render him incapable of being guilty of murder.

Defence lawyer Rochelle Mainwaring said the only issue in the case was mental illness.

Ms Mainwaring said the case was ``sad and tragic''.

``It is strikingly clear Mr Soares was so mentally unwell his behaviour fits into the insanity provisions of the law,'' she said.

Mr Soares brother Alex gave evidence this afternoon and described the heartbreaking circumstances leading to his mother and step father's death.

The trail continues until 4pm.