Welcome to the Bunnings Workshop community. It's fantastic to have you join us, and thank you for sharing your question about fence pailings.
I suggest using 100 x 15mm 1.8m Fence Paling H3 Hardwood as it is chemical free. It can be used for your shed, pet house and as side wall cladding. I definitely recommend that it be painted to add extra protection to it and extend its service life. I also recommend using Merbau Panels as they have a much better finish and are safe to use outdoors.
Would it be possible for you to post a photo on some of your proposed projects? This will give our members a chance to see what you are planning to do. We can then make recommendations on what materials to use.
If you have any other questions we can help with, please let us know.