Hi all, I had great success with Picaridan on the AAWT above Baw Baw and those bl----y monstrous flies, I suppose you have used it but thought I would mention it just in case. The 20% concentration "Off" brand and I did not get one bite or have one fly land on me. You must check the Off brand, not all is Picaridan and you want 20% it is in the smallish spray bottle -decant into a 30ml bottle -perfect. A week before at the same places I used a Deet blend (forgot the Picaridan!) and flies, as you would expect, landed on me. I applied the Picaridan "Off" in a very small amount rubbed well in only to exposed areas and the 8 hours protection suggested was very close in hot and humid conditions and a fair amount of sweat. The flies really just droned around me then the mozzies in the gloaming. Really excellent stuff! Highly recommended. Cheers.
PS I absolutely concur with the heat stress evaluation. This sounds exactly correct. I have experienced this also, in much the same way and it was the heat and my inability to cope with it- sip water slowly until you have rested for about half (circulation stabilises as stated) an hour then up the intake gradually-my solution was to buy a UL (130gms!) trek umbrella from Montbell for the prolonged sunny periods -Highly recommended also.