The man who brutally beat his 69-year-old neighbour to death and then, as she lay dying and unresponsive, sexually assaulted her twice, has appealed his life sentence.

On May 26 Jaden Lee Stroobant was sentenced to preventive detention and life imprisonment with a minimum non-parole period of 17 years for his "callous and depraved" crimes.

He had earlier pleaded guilty to murdering the 69-year-old Cunxiu Tian in her family home in January last year.

Justice Graham Lang sentenced Stroobant to life in prison for the murder with a minimum non-parole period of 17 years for Tian's murder.

For the sexual offending, which he said Stroobant has refused to properly recognise, he imposed a sentence of preventive detention.

The Herald can reveal that on June 13 Stroobant's lawyer filed an appeal against the sentence.

A date for the appeal has not been set.

Stroobant's lawyer Emma Priest confirmed that she had filed an appeal against the sentence of preventive detention which was imposed on the two counts of sexual violation with an minimum non-parole period of 10 years.

"The exact grounds are still being formulated, but in short the imposition of PD was out of line with all other cases and the exercise of the discretion was wrong in this case as a result," Priest told the Herald.

"In particular, his age, guilty plea and that he had never been offered substantive rehabilitative opportunities as his longest previous sentence was six months imprisonment.

"The exact details... will be expanded upon on appeal.

"The result was a manifestly excessive sentence so far as the PD applies in our submission."

Priest said the sentence for murder is not being appealed.

-NZ Herald