You can only go over it not behind it. .
How To Get Behind The Waterfall In Mt Coronet On Pokemon Platinum?
As far as I'm aware you can't get BEHIND it, however using waterfall you can go over it. If you use the waterfall method you can get the lustrous and adamant orbs
You just go through the cave to the right of ogboghrg or watever its cald wene you git to the cave keep going untill you get out you use rock clime and go back in the cave fron there then you will see the waterfall use surf and your right next to it but you will need the hm move to get past the waterfall