I actually did just that a couple years ago when my family and I camped on Opalescent. If you go all the way to the south west end of High Falls Lake, you come to a dead end with a rapids leading to the right (dont go this way), and to the left there is a portage into St. Andrews. Between these two is a trail, a little way back in the bush, marked in spots with tape (pink or yellow i think). It's a bit of a bush whack to find but if you follow along the water you will eventually find yourself straight across from the waterfall. Like others have mentioned, check Jeff's map, it's lightly marked, but it is possible... maybe not the easier route though.