New Zealander Blazej Kot has been sentenced to 25 years to life in prison for fatally slashing his wife's throat on a park trail in the United States last year.

The Cornell University doctoral student was convicted in April of killing 28-year-old Caroline Coffey, a postdoctoral researcher at the Ivy League school in upstate New York, in June last year.

He was sentenced today (NZT) for second-degree murder, third-degree arson and tampering with evidence.

Judge John Rowley sentenced Kot to 25 years to life, the maximum sentence on the murder count, with year-long sentences for the other counts to be served concurrently.

Prosecutors said the 25-year-old from Auckland plotted the killing and set fire to the couple's apartment to cover up the crime.

Assistant District Attorney Andrew McElwee told the court the murder was premeditated.

Kot's lawyer Joe Joch said he suffered from a mental disorder that made him think his wife had been replaced by an impostor.

He was mentally ill both at the time of the murder and now, Mr Joch said.

Shackled by his hands and feet, Kot expressed remorse at his sentencing, saying he loved his wife deeply.

"We had so much in common, and we had a wonderful future planned together," US media reported him saying.

"I still can't understand what came over me to do this."

He said Dr Coffey's death was a tragedy, partly caused by his failure to seek mental help, and apologised to her family and friends.

"The tragic fact is that I did this terrible thing, and there's nothing I can do to undo it," Kot said.

"The only thing that matters is that Caroline is gone. I'll have to live and die knowing I'm responsible for that."

Dr Coffey's father Michael placed a casket of her ashes on the prosecution table, telling the court her family "thought it would be appropriate for this convicted murderer to see what is left of his beautiful wife. That's all there is".