Luke Delphin, 20, gave 6ml of the party drug to Alesha Fernando, who he believed was 17, in June last year.

The pair then fell asleep on a mattress in Delphin's apartment.

When Alesha's friend tried to wake her, she was pale, her lips were purple and she wasn't breathing.

Delphin woke up and said: "I can't lose my apartment over this shit. It's not worth it".

Alesha remained unconscious and died the following day.

Delphin fled after Alesha overdosed and resisted arrest, punching and spitting on a police officer.

He has a mild intellectual disability and an acquired brain injury from substance abuse since he was nine years old.

Delphin was jailed for three-and-a-half years, with a minimum of two years, in the County Court on Wednesday.

He has already served 346 days in custody.

But Alesha's mother, who was visibly upset in court when Delphin was sentenced, says he should have been charged with manslaughter or murder.

The heartbroken mother spent her 34th birthday watching as her only child's life support was switched off.

"Nothing will bring Alesha back," Katrina Gavan told reporters outside court.

"She was beautiful, happy. She was too trusting of everyone. She was absolutely adored. Everyone loved her - took advantage of her."

Australian Associated Press