Welcome to the Bunnings Workshop community @TinyTouch. It's wonderful to have you join us, and many thanks for your question about securing pallets.
If you could tell our helpful members a little more about your project, they'd be better placed to offer helpful solutions. What's the purpose of these pallets? Are you making an aesthetic screening, or are they retaining something? How is the steel rod running through the post going to prevent you from fixing to it? What are the posts' sizes, and what are they made from? Any images you could provide would also be helpful.
Is there a chance that you could still put some fixings in the post despite the rod running through it? The easiest option will be running a couple of horizontal framing sections of timber between the posts and then screwing the pallets to that. Alternatively, perhaps you could concrete a couple of 90 x 45mm vertical sections into the ground and slip the pallets over the top. I've created a rendering below to illustrate.