Recently I flew with Jetstar, for checking carry on baggage, they required put your carry on baggage and you handbag including your laptop!!!
Normally a laptop around 2 kg, a carry-on 2 kg and a handbag or a laptop bag 0.5-1 kg, a magazine or book 0.5 kg, if you bring a bottle water 1 kg. A passenger need put all your change clothes on your body. Otherwise, you need pay $60 for a on site check in even you already have a check in bag much less than 15kg.
I suggest don’t fly with Jetstar. They trapped Custmoers first and then just rob them.
Domestic
Australia-Sydney
Australia-Melbourne - Tullamarine