It is complicated and to be honest I really dont know what the rules and regulations are so would have driven by and thought nothing about him dredging. Second to last time I was up at Arrowtown I noticed a fellow setting up a small dredger in the public fossicking spot so this chap cannot be a one off.
I think that if a chap knows the rules and regulations then a simple approach to the dredge owner might be enough to enlighten him but I would also say tha tin view of the trouble he has gone to you will overlook his transgression today but it might pay in his best interests not to return with the dredge in future otherwise the authorities will be notified.
As for me - even if I did see him sometime in the future I would say nothing because he is in HIS country and harming no one and it is only the unfair rules and laws which go against the natures and interests of good old New Zealanders that make it unlawful for him to do his dredging - the rules of course being in favour NOT of us but of the giant offshore companies who will one day want to put a huge strip mining machine in there, tear the sides of the river and the bed of the river to shreds, alter the landscape for ever, take the gold recovered off shore and leave our country a mess...their idea of reinstatment being to turn the beautiful rock outcrops and Tors into paddocks and mundane, boring shadows of their former glory.