Its hard to put an exact year on it, how do we define full equal major? Is it purely having the "top players" show up and play, having very in depth draws, or what. If you went by the entire top player argument as many do well I really couldn't see it as equal much before 1990...maybe like 1988 if you want to stretch it.
As for the late 60's and early 70's, it was seen as a big event because many great players at the time were australian and of course they played their home event, it was sorted of helped by the coincidence of some of the great being from there. In terms of worldwide equality though, I wouldn't say it was equal much before 1990.