Scott Guy was gunned down in the driveway of his Feilding home in July 2010. Macdonald, who is married to Mr Guy's sister, was charged with murder but acquitted by a High Court jury.
Macdonald admitted arson and causing damage to Mr and Mrs Guy's almost-completed new home in a bid to scare them. He smashed holes in walls, broke all of the windows and left obscene messages scrawled on walls.
Mrs Guy has said she wants to stand up for "Scotty" and Macdonald's other victims and make sure the suffering and distress he caused them is acknowledged and reflected in the sentence he is given. Protecting her young sons Drover, who she was pregnant with when her husband died, and Hunter is also important.
She told the Attorney-General: "If the offender is granted a concurrent sentence it will not reflect the suffering he has caused to my family, neighbours and the community and will mean that he is able to escape the true gravity of his actions."
A cumulative sentence would mean Macdonald served his individual sentences end-on-end. A concurrent sentence would see him serving time for each offence simultaneously.
"He needs to pay for all of his individual and separate crimes and the hurt, pain and suffering he has caused - not only to myself, my husband and our children, but to numerous other people within New Zealand," said Mrs Guy's letter.
Sensible Sentencing Trust spokeswoman Ruth Money said she supported Mrs Guy's call: "If I drove from work to home and got three speeding fines on the way, I would have to pay three fines - not one."
She feared a concurrent sentence would reinforce to other offenders that they got "free hits" after the first event.
Macdonald has been in custody since his arrest in April last year. The longest sentence he could be given is 14 years - the maximum penalty for arson. He may be given a reduction for his early guilty plea and time served in prison.
His accomplice, Callum Boe, a former worker on the Guy family farm, was sentenced to 19 months after pleading guilty to the same charges.
A spokesman for the Attorney-General said he had not seen the letter and it would be improper to offer any comment on a case before the courts.
Cumulative vs concurrent
When sentencing on multiple charges, a judge can impose cumulative or concurrent sentences.
Generally, cumulative sentences of imprisonment are handed down for crimes which are different in kind, whether or not they are a connected series of offences. Ewen Macdonald's offending would fit in this category.
Concurrent are appropriate if the offences are similar and connected.
High profile lawyer Nigel Hampton, QC, said a judge must consider a number of factors when deciding which kind of sentence to impose.
These include when the offences occurred, the overall nature of offending and any relevant relationship between offences.
Judges will also look at any factors that "aggravated" the offending or gave some kind of reasoning for it.
A court must ensure sentences reflect the seriousness of "each" offence. If a cumulative sentence is imposed, it must not result in a total period "wholly out of proportion to the gravity of the overall offending".
