A Sydney woman who was found to have killed her partner of 18 years with a euthanasia drug has had her manslaughter conviction quashed.

But the NSW Court of Criminal Appeal has ordered that Shirley Justins face a new trial.

In November 2008, the then 60-year-old was sentenced to 22 months periodic detention after being found guilty of the manslaughter of former Qantas pilot Graeme Wylie, 75.

He died from a lethal dose of the barbiturate Nembutal at his Cammeray home in Sydney in 2006.

The jurors were told Mr Wylie, who had Alzheimer's disease, was rejected for a legally assisted suicide in Switzerland months before his death, on the grounds he did not have the cognitive capacity to consent.

A long-time friend of Mr Wylie, Caren Jenning, 75, who travelled to Mexico to buy the drug that killed him, was convicted of being an accessary before the fact of manslaughter.

Jenning, who was suffering terminal breast cancer, killed herself using the same drug in October 2008.

In quashing the manslaughter conviction on Thursday, the Appeal Court found the trial judge had erred in his directions to the jury.