MELBOURNE, Australia — An Australian mother twice convicted of killing her two young sons was sentenced to 27 years in prison on Wednesday for what the judge called "the greatest act of betrayal."

Donna Fitchett was first convicted in 2008 of murdering her sons in 2005 and sentenced to 24 years prison. She appealed her conviction and was granted a retrial in May, but a jury again found her guilty after she admitted drugging her sons and then strangling one and smothering the other.

The boys were 11 and 9 years old.

"You were their mother. Your responsibility was to nurture, care for, love and protect them and over the years you did that," Justice Elizabeth Curtain said in sentencing Fitchett. "But in the greatest act of betrayal and in a profound breach of (trust) you robbed each of them of their precious lives ... in an act of unfathomable selfishness."

Fitchett claimed she was not guilty by reason of mental impairment but Curtain called the murder premeditated, citing a letter Fitchett had written before the murders.

No details on the letter were released, and a motive was not given during the trial.

Fitchett's lawyer, Patrick Tehan, told the court that the mother was depressed and suicidal when she killed her children.

She must serve at least 18 years before she is eligible for parole.

The boys' father, David Fitchett, said he had hoped his ex-wife would receive two life sentences.

"Thomas and Matthew deserve justice," David Fitchett told reporters outside court. "Today's sentence has been extended from the first one. Whilst in my mind it is nowhere near enough ... it is better than what it was last time. My boys, I love them and miss them."