Here is a list of gear you should take on the trip:

Passport

T-shirts
Light weight sweat shirt
Light weight coat/windcheater
Shorts
Light weight trousers
Under wear x ?
Socks - needed for some walks
Shoes - suitable for walking in the forest
Shoes - suitable for rubber tubing/water shoes ... can be hired for $5
Jandals/sandals
Swimming togs
T -shirt/rash top for snorkelling
Beach towel
Towel
Pyjamas
Light raincoat
Sun hat
Sun screen
Sun glasses
Insect repellent
Small personal first aid kit ... anti-itch, plasters, pain killers
Small torch or head torch
Tooth brush/paste
Soap/shampoo etc
Plastic bags ... for dirty/wet clothes

Spending money ... only for snacks and souvenirs.
.... plus if you want to do the optional Scuba diving the cost id A$50 for the first dive, then A$60 for a night dive.

Light backpack ... for day walks/outings and to take on the overnight reef trip (no big cases allowed)
1 Suitcase/bag

Remember that there are restrictions on liquids that can be taken onto the aircraft in carry-on luggage.

Optional extras:
Camera
I-Pod
Mask and snorkel

The overnight trip on the Barrier Reef requires you to only take one small bag. No shoes are to be worn on the boat.