I'm lucky enough to fish in waters where the Dusky flathead reside and they are the largest of all flathead (not Tiger flathead.....they only get to around 3kg). The duskies get very large (reportedly over 12kg).
One thing you will need to do to get constant success is make sure the fly bumps the bottom. This is why (as Harro suggested) the clouser is best. The upright hook helps avoid debris and snagging too.
If you can puff sand on the retrieve you are going to entertain more of the flatheads senses than just sight. His lateral line will be picking up vibrations as well as his sight........a rattling clouser is not a bad idea either. Flies that contrast the bottom are best as they are not very fussy at all.
The larger duskies around our way are usually in deeper water and never far from rock bars with good current breaks and back eddies. Interestingly too is the fact that in Spring we often catch the bigger girls in pairs.
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