EmpressRouge

Do you intend to use mostly public transit while in Australia or will you be riding in cars (including Ubers and taxis) a lot? And I'm assuming the carseat is one you bought in the US? One thing to keep in mind is that overseas carseats are technically illegal to use in Australia. The chest clip that is required in the US is not allowed in Australia. It's one thing to bring bulky car seat with the intention to use it at your destination, but it is a big hassle to bring it just for the plane ride.
https://www.childcarseats.com.au/faqs "No. Child car seats bought overseas do not comply with Australian Standards and they are not compatible with Australian vehicles."
https://travelcarseatmom.com/should-...-to-australia/

We are taking our very wiggly toddler to Australia in a few months (he will be 22M old), and initially I had planned to bring the FAA approved car seat we use exclusively for travel until I learned about Australian car seat rules. Technically, he will still be lap baby age, but he's agile and big to hold for that long of a flight. We were also aware of the QF rules against kids beds on planes (returning on the QF flight to SFO), so we just got a Kids Fly Safe Harness to secure him in his seat. He's already flown a 14-hr flight to Hong Kong, and was fine w/o a car seat on the flight. Our plan is to buy a convertible-booster car seat (the ones that last 6M - ~8Y) and ship and store it with relatives in Sydney. I know that's probably not feasible for most folks, but we intend to visit Australia/NZ a few more times in the next decade.

yno

We never travelled with the car seat in the plane. And when it was time to sleep, we just put the armrest up and they would sleep with their head on our lap.

willoL

I never flew without a child safety seat for either of my kids when they were infants/toddlers, and I never would. For me, the additional safety of the seat is more important than my desire to go anywhere with my kids, and my family is on the other side of the USA so I flew with mine regularly.

Infants have flown out of caregiver hands due to turbulence (though obviously that is very rare) and adults have died from turbulence-related injuries (also rare.) But, rare or not, my question is:
Could I live with myself if my baby flew out of my hands in flight and was permanently injured or died? No. Therefore, I always used a car seat.

I do believe most children (who travel routinely in cars) know and feel comfortable in their child safety seats, so there is an added benefit of providing a familiar environment to a traveling tot. That can improve behavior.

I did fly with my kids at least twice a year from infancy, but, at almost four years apart, the eldest was big enough for a booster by the time the younger one came along, so I didn't need to travel as a solo adult with two child safety seats. That would be VERY hard. I used a wheeled cart that attached to my Britax car seat to manage lugging (wheeling) the big, heavy infant safety seat through airports (as I flew alone with my kid/s), and I switched to a wearable harness called a RideSafer vest (as a booster-alternative for cars, not used in flight) for my older preschooler to reduce carry on luggage weight when we reached that point.

As an aside, I also wouldn't ever use an automotive child safety seat again if it had been checked (as luggage, under the plane.) I've seen baggage handlers toss suitcases around. A cracked child safety seat may not work as designed. (Child safety seats are meant to be discarded after ANY traffic accident above a certain speed impact.) Again, if my kid died because I used a seat that was damaged in transit, I wouldn't be able to live with myself. That's the standard I always apply to how I treat life-saving equipment, and how I make decisions on cost (of an extra seat for my kid to use a restraint) vs. my wish to travel.

It is 100% true that driving is more dangerous than flying, but it is also possible that a flight gets diverted and a parent with a small child could get stuck at an airport for lack of a seat to take a taxi to a hotel. I wasn't willing to risk having that happen to me, so I never flew without everything I needed to legally leave the airport with my young kids. That was another argument to travel with a child restraint while I had young children.

HfromIL

I totally agree with willol.

It�s not only much safer for your 2.5 year old to have a car seat, but more comfortable too. The getting to the plane can be tricky, but it will be 100% worth it once you're onboard. My guess is that you�ll find propping him up with pillows and blankets a hassle. On the whole, much more than a car seat. Having a car seat for him will count all the more as they are long flights. It�s familiar for him asleep or awake, and much more comfortable than the plane seat would be.

This is what I found when I went to Rome with my 11 year old daughter. The flights were 9 hours each way and she was in her car seat (Britax Frontier Clicktight) for the whole of both flights. It�s obviously a different ball game with a 2.5 year old, but I don�t think different enough to not bring their car seat. My daughter was much happier buckled into her car seat than she would have been on the plane seat, and she never once wanted to get out of her car seat. She was totally comfortable despite it being a long flight and much safer.

Carrying her car seat wasn�t easy but I borrowed a little fold up cart from a friend which I used to roll it along. The biggest hassle was that it had to be manually inspected at security as it wouldn�t fit through the scanner, but that was fine and no big deal. Maybe looking into getting a wheeled cart for his car seat might help.

Erasmus

You don't mention cabin of service, so I'm assuming coach. In that case, bring the car seat, no doubts about it. Unless he is quite large for his age, it will be the most comfortable option for him, and gives you the choice to restrain him so you can nod off from time to time.