Considering going travelling in a couple
This may seem like a ridiculous question
Do people actually just carry one back pack or do they bring a suitcase of stuff and store them in hostels along the way?
I am considering Thailand and Australia (open to more suggestions) from September onwards
There's always exceptions, but yes, most people just carry one backpack, and probably a small daypack along with it for a bottle of water, guidebook and camera. (For the big backpack, aim for something in the 50-60 liter range; the 70+ liter backpacks only tend to make sense if you're quite tall and fit and have specific needs.) You really don't need that many clothes and things - you need the same amount of clothes for a week as for a year; just wash them regularly. (People who prefer travelling with just a 30-40 liter backpack even tend to stick to just two sets of clothes, wearing one while the other is being washed.)
You'll probably even pack a bit less for a year than for a week. When going away for a week, you'll probably have lots of "better pack this just in case, I might need it if..." things. But when going for a year, you'll have to carry all those things all the time, so the decision to leave them at home is much easier. And if you end up needing one of them after all, you can just buy it while out there.
One hint: Definitely don't bring any of your favorite clothes. After a year on the road, being worn constantly, they'll be absolutely threadbare, and ready for throwing out.