First of all, anything that you might forget before you leave for your trip to Australia, you will easily be able find over there. If the worst comes to the worst, you'll be able to delve into your pockets and buy anything in Sydney or Brisbane during a day of shopping. It's obviously slightly exaggerated but it won't be dramatic if when arriving there you realise that you've forgotten a T-shirt or a pair of flip-flops. You won't need to make a 48 hour return journey to pick them up.
Regarding clothing, it is hot in Australia so make sure you have lightweight clothing which will help you to withstand the very high temperatures. Depending on the season, also take something to cover up when it gets a bit cold in the evening.
To protect yourself against the sun, you will need something to cover your head as well as a pair of sunglasses. Also, don't forget sun cream. If you don't like to apply it, take Biafine to relieve sun burn. But as you also need to apply it, opt straight away for protective cream.
It's not worth bothering with your first-aid kit. Simply take something to relieve minor injuries. In any case, you'll also find everything over there.
Crucially, you'll need your credit card for cash withdrawals. Also, ensure you have a photocopy of your passport, which is very useful in case of loss, and your international driving licence if you intend to hire a vehicle. Finally, and most importantly, bring your camera or a video camera to bring back countless memories! The seabeds are splendid near the barrier reef, if your camera is waterproof, it's great.