Outgoing Australian Greens senator says his biggest political mistake almost saw him ban lesbian sex as a lone Green MP in Tasmania during the 1980s.

In a farewell speech ahead of his formal retirement next week at the National Press Club yesterday (5 June), the first openly gay senator recalled how the then opposition Labour party's decision not to move and get rid of section 122 of the criminal code - which banned male homosexuality only - caught him off guard.