They will include it in the carry on allowance, 23kg on Qantas, 20kg on the others. So you should be well and truly covered allowance wise.

You can take them onboard sometimes I think, but honestly, they o not fit well as a car seat in an aeroplane seat.

We have taken our car seat on over a dozen flights all over the country and overseas and have not had a single issue with regards to damage, so there is really no need to worry about that from my experience.