I think you better talk to the pertinent water authority. Is this property in NSW or Victoria? Unless specified in land title deeds, land owners who have a river or stream running through or adjoining their property gave certain riparian water rights under the common law, but I am sure that in the case of NSW if they give you permission (get it in writing - friend or not) then any proceeds from the fossicking are yours. Refer to the NSW Mining Act. It. My understanding that unless your friends possess an Exploration Lease, an Assessment Lease or a Mining Lease that covers their property and they have given you lawful permission then any find is yours. Also review the Fisheries Management Act, Native Title act and the Protection of the Environment Operations Act so you better understand your legal rights and obligations. Basically, unless specifically covered and lease or licence the Crown owns everything underground.