I just travelled with Maddy to Sydney from Brisbane for two days last week. All of the airlines for interstate travel do not charge children under 2 for a seat. Although they do not get their own luggage allowance; any paying adult can trade in 5kgs of their 20kgs allottment in order to bring along any number of things for baby/infant. ie. high chair, pram, car seat, portacot. There is no weight cut-off for these baby items once you have traded in your 5kgs. You are also allowed to bring along a changebag on plane (limited to the usual 7kg allowance) for the child.

Given how much of Maddy's bag (that went in with the usual luggage) was taken up by nappies (we use disposables), I would suggest you take enough for the plane trip and for immediate use once you arrive in Brisbane; then buy nappies at the first grocery store you find. Same deal for the formula; although perhaps you could buy a couple of those single serve formula sachets just in case?

Do you know where you are going to in Qld? We have both a high chair & bouncer that you can borrow for Alex if you'd like!

Oh and I'm sure plenty of people have mentioned previously, but breast feeding on take off & landings on planes reduce the air pressure problems on baby's ears. Maddy didn't make a peep with this approach.