Personally, I don't think the risk of contamination is that high if you are lining it with builders plastic. I think there would have been an issue years ago, but now? not so much. But then again, I am not an expert, just someone who wants cheap and easy garden beds.
Welcome to the Bunnings Workshop community @Pompomz. It's great to have you join us and many thanks for jumping into the conversation.
I trust @lcantem will appreciate your kind words about their project.
I'd encourage you to start your own discussion if you need assistance with your project or would like to share it with the community. If you need to source some free pallets many of our stores conveniently locate them for customers like yourself to collect. I get my pallets from my local Bunnings and they always have them stacked up and ready to go.
We look forward to seeing your project come together and please let us know if you need assistance with it.