I love how they have their spare boxes to take as you please as I use them to post items in but:
1) do employees care if you take some without buying anything?
2) is there a certain day of the week that there's more or less boxes put out or is just pot luck when stock gets put out?
If you work at any of the Bunnings in the South or South-East of Melbourne I'd love to chat with you via PM.
Each Bunnings store will be a little different. Where I work in Western Sydney, we don't mind if you take boxes without buying something. You may take as many as you like. The box bay gets decimated on the weekends and doesn't tend to fill up again until Tuesday.