I hope I can help hon. I've done a lot of travelling and I was a travel agent and still have a SIL in the industry so hopefully this helps!

Infants can travel free in their car seat providing you have paid for an extra seat on the flight. Unfortunately, I don't think toddlers are covered in Australia yet. They're expected to sit in the normal aircraft seat and their carseat travels with the luggage but phone the airline to be sure.

Normally in Australia, you would check your car seat in. We always fly Qantas and check our car seat in at the check in desk then it's taken to oversized luggage - depending on the airport, some times we take it and sometimes the staff take care of it. They've always wrapped it in plastic for us and it's never been damaged.

You would definitely need to purchase normal 'check in baggage' seats if you want to checkin a carseat. The carry on baggage only fares are VERY restrictive and you will be charged ridiculous amounts to take your car seat as 'excess'. Ph the airline before booking to be sure and get any confirmation of a 'good deal' in writing - an email or proof on their website.