Before you go through customs, when you arrive in Australia, you are very likely going to collect your hold luggage and as such it does not matter where you transport your things.
But with the risks of luggage being lost and things going missing out of luggage, as well as the fact that airlines and most insurance companies do not pay for valuables in checked in luggage, you better keep all you value in your carry-on.
Whether you need to pay duty or tax depends on the rules for your arrival country and how much other items you bring, and on whether you are a visitor who brings those items in for a short time or a resident or citizen returning home.
For about US$300 you do not need to declare on short stops, but if you go out of the airport at either of the stops you will have to pass through customs. In that case you may look for a storage (locker or left luggage area) in the international part of the airport and check that you can go to that part of the airport on your return.
The safe option is to declare them when you arrive in Australia.
But all countries I have ever traveled to have a 'what and how much can I bring back home' on their websites. And if you stay under that amount, and they do not contain items on the 'not allowed list' you can just walk through 'nothing to declare'.
I did not find the actual amount you can bring in as Australian, just that you need to declare valuables on your 'incoming passengers card', see this page for more details.
@jcaron did find for you that now, 2018, Australians are allowed to bring in AU$ 900, see this page for details.
(Remember to check the amount if you read this answer in a later year, as they do tend to change over the years.)