Coroner Nagara said she had been mindful that an inquest had been held into the 2006 deaths of infant twins Chris and Cru Kahui after their father, Chris Kahui, was acquitted of their murder.
Coroner Garry Evans, who conducted that inquest, found he was the only person present when the babies suffered their fatal injuries.
Coroner Nagara said that case differed because the possibility of other suspects had been left open, and their names could be cleared at an inquest.
"This is not the case here,'' she said.
'"... There is no cogent evidence pointing to anyone else having been involved, so the finding of fact implicitly urged is that Mr Macdonald was the person who killed Mr Guy.''
Coroner Nagara said she had conveyed this provisional view to Mr Guy's parents and to his widow, Kylee Guy, earlier this year.
Mr Guy's parents took no issue with the coronial inquiry not being resumed.
Kylee Guy, however, had sought through her solicitor Chris Morris to resume the inquiry.
Mr Morris submitted to the coroner that the circumstances of Mr Guy's death had not been established and ``the identity of the killer is a question of fact directly relevant to the circumstances of the death that has not been determined in the criminal proceeding''.
He said Macdonald's acquittal did not render it inappropriate for the coroner to establish who inflicted the wounds to Mr Guy.
"The question of who factually killed Mr Guy is quite a different question to that of whether Mr Macdonald bears any legal responsibility for his death,'' he said.
Macdonald is currently in jail for serious criminal offences against the Guys and others, including arson.
Mr Morris said the only information about Macdonald's whereabouts at the time of the killing as it stood was contained in police interviews, and a coronial inquiry would be an opportunity for him to be summonsed to give evidence in person.
Mr Morris said it would be in the public interest for there to be a factual finding about who killed Mr Guy, and that Kylee Guy felt strongly - for her and the couple's two sons - that they be assured all available steps had been taken to find out how he died.
Coroner Nagara, however, said that because the killer's identity had not been established, it did not mean the circumstances of death had not been adequately established.
"It is the identity of the deceased that needs to be established. It is not a requirement that a coroner should necessarily establish the identity of other person(s) involved in the death,'' she said.
Resuming the inquiry would risk the Coroner's Court becoming a "back-stop jurisdiction for the reconsideration - by a different process, by different rules of evidence and to a lower standard of proof - of criminal matters, which is a function fundamentally and uncomfortably at offs with its primary purpose''.
July 8: Scott Guy shot dead in the driveway of his Feilding farm
July 16: Mr Guy's funeral held
July 20: Police reveal the Guys' home had been subjected to damage and graffiti
September 16: Mr Guy's wife Kylee gives birth to the couple's second son, Drover
April 7: Ewen Macdonald, married to Mr Guy's sister Anna, is charged with murder
June 5: The murder trial starts in the High Court at Wellington
July: Macdonald acquitted of murder
September: Macdonald sent to prison after admitting other serious offending against the Guys and others including arson
December: Macdonald denied parole
