I fly QANTAS once a year, and I've always taken on a carseat.
You MUST call them before you leave, about 3 weeks, and give them the details of your car seat. They want to know the company that makes it, the model, the widest point of the car seat (I lied and said 16.5" when it's 17.5"), how many harness points (5), does it have a top tether (don't know why they ask this as it can't be used on a plane) and where it is made...I think that is all they ask. The customer service rep will record the information and then the request goes to Sydney for approval. Sometimes they call back and tell me it's been approved, other times I have to call and ask them. I'm pretty sure you'll have to install the seat forward facing - Australian laws forward face at 6 months I think. I always kept DS RF in the car when we visited.
When you get to check in, you have to show them the "approved for aircraft" sticker, so know where that is, it took me a couple of minutes to find it the first time. When you get to the gate tell them that you have had the seat checked at check in. They might asked you again as you board the plane. Coming home from Brisbane, the check-in and gate staff all said that yes it was approved, but it wasn't written on the boarding pass and the flight attendant wasn't happy about that. The gate staff said it was approved, and she made them go back and get a print out from the computer. While that was being done, she did let us install it - she believed it was approved, but I think she wanted the paperwork to cover themselves.
Flights to Australia leave at 11pm or so, and they serve dinner about 2 hours after takeoff, and then breakfast is about 10 hours later, so you're eating breakfast at Australian time. They do give out snacks. We always eat dinner at the airport before boarding. We then go straight to sleep and don't eat their dinner. DS normally wakes up before the breakfast is served and he's hungry, so I have plenty of snacks with me to tide him over until breakfast is served. Ask for the kids meals, they're pretty good.