Michael Kirby testified in favor of same-sex marriage at a hearing in Sydney this week, the Sydney Morning Herald reported.
He said he is a "second-class citizen" because there is no public recognition of his relationship with Johan van Vloten, who has been his partner for 43 years.
Kirby, who served on Australia's highest court from 1996 to 2009, came out in 1999 by naming van Vloten as his partner in his Who's Who entry. On the court, he was known for frequent dissents.
Kirby said he does not see how acknowledging gay relationships harms heterosexual ones.
''A loving relationship of tenderness, of gentleness and affection, and fidelity and support is a beautiful thing and anyone who would disrespect it is not a kind person,'' he said.
Three bills have been introduced in Parliament that would legalize same-sex marriage.