The 1998 ARIA Rock and Roll Hall of Fame inductee had been battling myeloma for some time and had been on life support since Monday.
The Masters Apprentices formed in Adelaide in 1967, rose to national promise and disbanded in 1972 after unsuccessfully trying to crack the UK market. Their songs "Turn Up Your Radio", "Because I Love You" and the snarling "Undecided"" (a garage rock classic) are embedded in the history of Australian music.
The Masters reformed in 1987 and numerous times since. In recent years, Keays had been gigging with contemporaries in Cotton Keays & Morris.
Noise11 website, who broke the news, lauded Keays as "one of Australia's first and greatest rock singers".
Masters Apprentices songs have been covere3 by the likes of Hoodoo Gurus (who collaborated on a single of "Turn Up Your Radio" with Jim) and Radio Birdman (who played a fiery version of "Buried And Dead" live on their 2002 Australian tour).
After the dissolution of the Masters, Keays forged a successful solo career and to the surprise of many in 2012 he returned to prominence in Australia with a CD of hard-edged garage classics, "Dirty Dirty".