Take a change of clothes for him at least (and if he's a spewy bub like ours, take a new shirt for you...) on the plane with you - he'll probably choose the worst possible moment to be sick all over everything...

The most useful thing we've found to take when travelling with our 6mo is a packet of disposable change mats. I know: the landfill component is awful (though no worse than disposable nappies, which we are also using as we travel), but it's seriously handy to be able to put her down on someone else's sofa/bed/rug/whatever, with nappy off, for some nappy-free time, without having to worry what she's going to do to their furniture. I lasted about a week just washing our waterproof mats daily, but when we were in a hotel and couldn't wash so easily the disposable mats were a lifesaver. I think you can get them at supermarkets in Australia, but not certain as I haven't bought them there.

What else?

I believe that the airline will make you check the stroller through. They will, I believe, take it as baggage at no cost to you (they'll take a stroller and a carseat, I think - check the fine print on their website). I suggest you might want to have a carrier (a sling or something) to carry your baby on the plane and in the airport after you check the stroller (not sure what a Kozy carrier is - it might do the trick perfectly - we use our Ergo).

Do you have a carseat? I don't know what the law in in Tassie, but I'd guess that you can't get a taxi from the airport without a carseat. Also, if you want to be able to take both your baby and your friends baby in a car together, you'll need two.

Take some familiar and some new toys for the plane. You may have to distract him if he's upset by something. Try hard to time it so you feed him on the way down on each flight: this will help him to adjust the pressure in his ears by swallowing, and prevent him from having painful ears and screaming during the descent. We've had no trouble on any of the 5 flights that DD has taken (from 3-6 months old), but there have been babies on the flights who, listening to them, were pretty miserable about it.

it's going to be fine. Staying with a friend who understands about babies that age I'm sure you'll have no hassles. Hope you have a great trip!