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Hi and thanks for taking a look at my question 
I'm wondering firstly what is the best way to fix up what's here and secondly, on-going maintenance
Cheers,
Rob
Welcome to the Bunnings Workshop community @rjurd. It's marvellous to have you join us, and many thanks for your question about maintaining timber stairs.
Typically you would apply a yearly coat of oil to repel and prevent rot. It looks like this might have been skipped for several years, and the timber has started to decay. Due to the level of decay on the top two steps, I recommend you consider replacing the treads. The bottom one looks like it might have been replaced somewhat recently (or flipped upside down as indicated by the inverted stain drips) and only needs sanding and recoat of oil. Alternatively, you could replace the lot with a Stringer H3 Treated Pine Suits 3 Tread.
If the top two treads are structurally sound, you could sand them back with 120-grit sandpaper to remove the weathered surface timber and coat them with oil. However, I'd err on the side of caution and look at replacing them as they are potential fall hazards.
When sanding your timber, remember your PPE, including a dust mask, safety glass and gloves.
Please let me know if you have any questions.
Mitchell
