"Hard Yakka" is Aussie slang for describing hard physical labo(u)r. For example, if on the weekend I had to put in a new fence in my backyard, I might say to my colleagues something like "Digging the holes for those fence posts in the %^&#$ clay soil was bloody hard yakka, I can tell you."

I am not sure of the origins of the expression, but there has been a range of working clothes, made here in Australia for decades, called "Yakka". I don't know what came first, the clothes or the expression. Of course, their advertisements for their work clothes show a whole lot of big, fit, strong men wearing their "Yakka" work clothes, pulling mining equipment, digging holes, or whatever, in the hot Australian sun, while there is a chant of "hard yakka, hard yakka".