Welcome to the Bunnings Workshop community @aj88. It's brilliant to have you join us, and many thanks for your question about cleaning timber.
I recommend that you give them a scrub down with Feast Watson 1L Woodclean and a stiff bristle brush. This product will remove the discolouration and return the posts to looking like new. You'll then be able to apply the stain of your choice. By preparing the timber, you'll avoid any discolouration through the final semi-translucent stain. Remember to wear PPE when using the timber prep solution, including safety glasses, gloves, face mask, long-sleeved shirt and pants and enclosed footwear.
If you were painting the posts, you should only need to give them a quick sanding before applying your coating.