McDonnell admitted to killing her sons, 10-month-old Jayden and 2-year-old Connor, in a bathtub in Canada in 2010.
During the trial, she said she was severely depressed and suicidal when she drowned the boys.
McDonnell and her husband Curtis were in the midst of a messy divorce at the time of the killings, and prosecutors alleged she was attempting to get back at him.
In April, McDonnell received an early release from a psychiatric hospital after serving 10 months of a six-year sentence.
She was deported back to her home country of Australia, the Australian Broadcasting Corp. reported.
Her body was found lying on coastal rocks in West Gosford early Wednesday.
Police said an autopsy would be conducted, but authorities don't believe foul play led to her death, the Sydney Morning Herald reported.