Cook What to Pack

Luggage and bags: Luggage up to 20 kgs can be checked at the station before departure, but this is not obligatory. It is technically possible to store two big suitcases in a sleeper compartment underneath the lower berth, sacrificing some leg room (we did it but lesson learned). As these compartments are narrow, my recommendation for two people travelling together is this: pack everything both of you will need for the journey into one bag/suitcase and check the second suitcase, you'll be more comfortable.

Clothing/Shoes/Weather Gear: Have a sweater or cardigan and (ladies) a cotton or silk scarf at hand. The air condition especially in the lounge and restaurant car is notoriously set at too low a temperature and there is an unpleasant chilly draught from the ceiling.
Have a sun hat for the stop in Cook.
Dress code: there is not really a dress code, you don't have to bring your Sunday Best, but I'd recommend not to wear your Weekday Worst either.

Toiletries and Medical Supplies: Passengers in Gold Kangaroo Class receive a complementary tolietries bag which contains a toothbrush and toothpaste, shampoo, conditioner, shower gel, moisturizer, re-hydration spray, a washcloth, a sewing kit, a shower cap, and shoe polish. So these items do not have to be carried. Towels, bedding and sheets are provided.

Photo Equipment: Yes you will want to have a camera.

Miscellaneous: No need to carry any food, you will be fed until you almost burst. A bottle of water or two to have in your compartment will be useful, maybe a few sweets or fruit if you must.
Note that interstate quarantine restrictions on fruit, honey etc. apply. There will be a control on the train before arrival in a new state, so finish all your apples or whatever in time.

Footnote: These tips apply to Gold Kangaroo Class, i.e. first class sleeper, only. People who travel in Red Kangaroo or even daynight seater will have to pack differently.