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August is typically the "coldest" time of year in Qld, so the bugs won't be as plentiful as other times of the year. Having said that, particularly up north there will still be plenty and you shouldn't have to go looking - they'll come to you. I'm up north and have both the 100L macro and the MP-E 65 and don't use the MP-E very often for outdoor shots. The northern critters seem to be very sensitive to the pre-flash, so I usually make do with the 100L. It depends how close you like to get. Unfortunately the most common lizards and amphibians are the imports (Malaysian Barking Gecko and Cane toad) and they are very, very plentiful, though will be scarce at that time of year down south. You can usually find some native geckos amongst the imports, but they are rather shy. In my experience you have to be lucky with frogs or go looking. As for colourful insects, I believe MarkB1resides in Brisbane
and has an excellent gallery of what's available in the southern regions at various times of the year.
While in Cairns I can recommend the Kuranda butterfly sanctuary if you want some shots of the Cairns birdwing and the Ulysses butterflies. You may also wish to try the Atherton tablelands where you should be able to see platypus in the wild if you go to the right spots at dusk or dawn. The faster 300/4 would be beneficial here due to the low light, also for the geckos when they appear at dusk. The saltwater crocs are plenty big enough for the 300/4 without getting too close. Certain spots (Paronella Park and places on the tablelands) are also good for freshwater turtles. For access to all sorts of wildlife places like the Canopy Treehouses on the tablelands can offer some great shooting opportunities and everything pretty much comes to you (except the tree kangaroos, in my experience).
On Fraser Island, the dingoes are typically more wary these days, so you may need the 300 for them. If you like spiders, there will be plenty to keep you busy. If you look carefully around Kingfisher Bay you may be able to find some Funnel-web spider burrows. I found one just off the track up to the lookout a long time ago. There also used to be one just outside the change rooms at Lake Mackenzie, but it was gone last I checked. You may also come across bull ants on Fraser. They are easy to spot at around 1", but hard to shoot because they are typically coming after you - and a cautionary word, they hurt...a lot!
I hope some of this info is useful. Enjoy your trip.