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Welcome to the Bunnings Workshop community @eteractive. It's sensational to have you join us, and many thanks for your question about using structural timber outdoors.
Typically you'll need H3-treated timber for exterior above-ground use, and for in-ground or on-ground use in moist areas, you'll need to use H4-treated timber. There is no treatment I am aware of that will turn internal structural-grade timber into something suitable for exterior use. You can coat the timber with exterior paint, extending its lifespan, but any water that finds its way to the timber will begin the decay process rather quickly.
This might be one of those cases where you should save the timber for some internal projects and purchase suitable timber that will ensure your project stands the test of time. If you're looking for a budget-friendly plant stand, I'd recommend this step-by-step guide on How to build a tiered plant stand. You could cut the cost even further by using sleepers for the shelves instead of the Cypress pickets.
Please let me know if you have further questions.
Mitchell
