De Silva appeared in the Christchurch District Court on Friday to hear his fate after a week-long jury trial ended in a guilty verdict last week.
He was sentenced to two years and eight months in jail and will be deported to his home country of Sri Lanka as soon as he is eligible for parole.
During the trial, De Silva maintained the sexual connection between himself and the victim was consensual.
The jury was told De Silva believed the young woman propositioned him when she came downstairs and was unable to wake up her comatose boyfriend after a night of drinking and smoking cannabis.
The victim had a different story and told the jury she was asleep in her boyfriend's bed when she woke to De Silva performing oral sex on her.
Judge Gilbert told De Silva he believed the prelude to the oral sex was a figment of his imagination. The victim had not given him any reason to believe she was interested in him.
De Silva has been in custody since 2019, time that could be taken into account as "time served", meaning he could be paroled and deported sooner.
Judge Gilbert told De Silva his offending had taken a great and understandable toll on the victim.
She wrote in her victim impact statement that two years after the attack she continued to suffer from high levels of stress and difficulty sleeping. She said her career had suffered and she struggled with relationships.
This is not De Silva's first conviction.
He was convicted and jailed in 2019 after pleading guilty to using 18 fake $50 and $100 banknotes to buy items including pants, socks, and stationery.