The Crown says David Bain, 37, killed his parents Robin and Margaret and siblings Arawa, Laniet and Stephen at the family's house in Dunedin on 20 June 1994.
The defence says Robin Bain killed the family present in the house before shooting himself.
Robert Chapman, who carried out post-mortems on Princess Diana and Dodi Al Fayed in 1997, has reviewed the pathology of the case.
He told the High Court in Christchurch on Wednesday the wound on Robin Bain's head was a contact or near contact wound, meaning the rifle was held very close to his head.
Asked to comment on the location of the wound, given that Robin Bain was right-handed but was shot through the left temple, Dr Chapman told the court that those intent on suicide will often choose unusual methods.
Earlier, three pathologists who gave evidence for the Crown said Robin Bain's wound was not self-inflicted.
One of the Crown pathologists, Dr James Ferris, said suicide was an "impossible" scenario as the bullet was fired from between 30 to 42 centimetres away.