Thank you for sharing your question about building a deck. I suggest looking at this guide - How to build a low-level deck by @Adam_W. This guide will give you the necessary information you need to plan your deck. I'm sorry, but Bunnings does not have a store-level workshop for framing and decking. However, they have recently released a seven-part series on how to build a deck.
In order for us to give you more recommendations, would it be possible for you to post a photo of the area where you plan to build your deck? Can you also please include the proposed size and shape of the deck? This will give our members a better idea of the terrain. We can then propose more suggestions on how to layout your deck.
If you have any other questions we can help with, please let us know.