Hi @swong1,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Unfortunately, I don't think bog is the answer to your problem. Considering the load-bearing nature of the timbers that you've provided photos for, I would suggest they need to be replaced.
The bearer in your first image is transferring the load applied to the landing into the post. If this is severely rotted, which it looks like it could be, then it is possible the landing will give out and collapse, which has the potential to cause harm to whoever is on it at the time.
The second image shows your post is rotted out at the level of the singular bolt holding it to the anchor. If this post is knocked, or a strong wind comes, it could potentially come loose of the anchor, toppling whatever load is being held above this.
My suggestion is to have this assessed for repair by a qualified carpenter. They will be able to safely remove and replace the rotted timbers.
Any remaining holes that are not detrimental to the stability of the structure can be filled with Builders Bog.
Let me know if you have any further questions.
Jacob
