Hi Nugget - I haven't personally tried my luck that way but grew up in Nelson and know the area pretty well. I used to know a couple of people that had claims on the Wakamarina goldfield and have always wanted to give it a go up there. I could be wrong but I understand that the campground up there at Canvastown has a claim for a stretch of the river and allows guests that are staying with them to prospect.
Like you, when I saw how much rain had fell up there I wondered if it might bring some gold down and cursed my bad fortune at being at the wrong end of the island!
Here is an article about people looking for gold in Canvastown after a much smaller flood last year
http://www.stuff.co.nz/m...stowns-second-gold-rush
If you do head up the Wakamarina way, watch out for the ghosts of the old miners.... In 1866 the Burgess Gang murdered five people on the Maungatapu track. Happy hunting!