that´s a nice trip. my last trip to Queensland is not too long ago and if you don´t want to get very close pictures of shy wildlife in the wild you´ll be fine with the 70-200 with extenders. I might get also the 1,4 III
I have the same lens on my 5D III and the pictures are better and the AF is quicker with the 1,4 III.
I have the 2x III with me if I need the extra range but perfer the 1,4 III.
This year we do a trip from Perth via Tasmania to Sydney for birding and I got myself a 600 II for this trip and we don´t have any luggage issues. We try to fly Jetstar as much as possible because you get 10 KG of carry on luggage and you can choose how heavy your suitcase will be.
I´m traveling all the way from Germany and the only issue we have is, that they weight even your carry on when you leave in Australia and for us once again in Singapore ( but they don´t have Problems if its 1 KG more than 7 )
I have all my equipment in a Lowe Flipside 500AW, even the EF600 fits in there nicely. It´s the size that is allowed in Australia for carry on, they have different sizes then we in Europe or the US and not 55cm x 40cm x 20cm. It´s the perfect backpack for a holiday, I walk around with it the whole day without any issues.
And if your Handluggage has to get weighted put your raincoat or what ever you take on the plan in the back of your luggage cart and hide your 5D III with the 70-200 in it. This should lower you handluggage weight enough to get through and not pay extra. Or put it in your wifes backpack, like I do most of the time
I don´t know if you are into snorkeling or diving but they have a wunderful underwaterworld and it might be good to get a nice underwater camera like a D20 not to expensive and big but get´s you nice pictures where you won´t take your 5D III.
Have a nice trip and hopefully you might post a few nice landscape or wildlife pics here.