Police initially thought 38-year-old Deborah Balbi died of a drug overdose, but a twist has emerged.
Ms Balbi was found dead in the Maylands townhouse in South Australia she shared with her longtime partner Michael Johnson in December 2012.
However, her family was shocked when 43-year-old Johnson was charged with her strangling murder some 975 days later.
Deborah's brother, Steve Balbi, said in August 2015, "Can't believe she's gone. She didn't deserve this."
On Monday, Johnson, a Salvation Army volunteer and devout Christian, pleaded guilty to killing Ms Balbi.
The judge was told Deborah Balbi's 86-year-old mother will travel from Italy to provide a victim impact statement when Johnson is sentenced in July.