Hi @terry78,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Assuming the posts are timber, and you are using 50mm thick sleepers, I'd suggest using these Buildex 100mm Climacoat Timber Construction Screws, if your sleepers are 75mm thick, I'd suggest Buildex 125mm Climacoat Timber Construction Screws. These screws are galvanised and coated to prevent reaction between any timber treatments and the galvanisation.
Use a Sutton Tools 5.0mm Long Series Jobber Drill Bit to drill pilot holes before driving in your screws.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
