Filmmaker Ted Geoghegan announced the Australian actor's passing and hailed him as "an unsung hero of Aussie cinema" -- adding he's "continually floored" Hugh played the main villain in both 'Mad Max' movies.

It's true -- Hugh became famous for starring opposite Mel Gibson in the 1979 film as the central antagonist, Toecutter ... then returned to face off against Charlize Theron and Tom Hardy as Immortan Joe in 2015.

As you'd imagine ... he's the only actor to appear in both flicks, and he and director George Miller had a very good working relationship over the decades. Miller also cast Hugh in the scrapped DC movie, "Justice League: Mortal."

That's not all he's known for though -- Keays-Byrne also starred in several British and Australian projects, including "Stone," "The Chain Reaction" and "Resistance."

He had more than 50 acting credits to his name spanning nearly 5 decades.

So far, Hugh's cause of death is unknown. He was 73.