Hi Nikos,
All that phantom83 said. Other places worthwhile are the central part of the river (near Harridge Falls) and the top area downstream of the pub once you start to get to bedrock. Up there be respectful: some people have been undermining the bank and eroding the ford where the cows cross. Easy way to piss off the farmer who has the lease, and there are plenty of other places to go. The sapphires are larger but fewer. You'll need a Prospecting Licence anywhere out of the Fossicking Area. If you feel adventurous, the lower Wynyford, the Frome, the Cascade, Main Creek and Black Creek can all be good in places.