Backo's family released a statement through the Canberra Raiders on behalf of the now 62-year-old thanking the public for their thoughts and prayers as Backo remains in a Cairns hospital.
"Christabel, Sam's wife and extended family appreciate the support and prayers received for Sam," the statement read.
"Sam had a heart attack yesterday morning and at this stage remains in a critical condition.
"The family has asked to please respect their privacy at this time."
The hard-hitting prop, who played 115 games for the Raiders, before also representing the Brisbane Broncos to the end of his career in 1990, played seven State of Origins for Queensland and six Tests for Australia.
Playing for Fortitude Valley in the Brisbane competition from the start of his career in 1980, he would make the shift to the Raiders in 1983, where he spent six seasons, before enjoying a brief stint with the Leeds Rhinos in the English Super League in 1988 and 1989.
Named in the Indigenous Australian team of the century, Backo was also awarded the Australian Sports Medal for his contribution to rugby league in 2000.