The former University of Otago economics tutor was found guilty of the murder of his former girlfriend, Sophie Elliott, and remanded in custody for sentence on September 15 in Christchurch.
Weatherston could expect to be sentenced to life imprisonment, but the question was how long the minimum non-parole period would be, Auckland University associate law professor Scott Optican said.
Under section 103 of the Sentencing Act, the minimum term of imprisonment must not be less than 10 years but "could be more", he said.
The court would need to be satisfied of a number of factors, including whether the sentence holds the offender accountable for the harm done to the victim and the community.
The court could also make an order under section 103 of the Act and impose a minimum period of imprisonment of at least 17 years in certain circumstances.
These included if the murder was committed with a high level of brutality, cruelty, depravity, or callousness, he said.
Preventive detention was not available for the crime of murder.
