Hi @Caz27,
Thank you for your question and welcome to the Bunnings Workshop community, it is great to have you with us.
Looking at the image, the water damage only appears to be at surface level. Since this is the case, the fix will involve sanding back the sill and recoating. I'd suggest removing all of the existing stain as if you just attempt to stain the affected area, it is likely to stand out when surrounded by the older stain.
If you are looking to retain the same look, I would suggest heading down to your local Bunnings with a few photos before sanding and speaking to their Paint Expert as they will be able to help identify a similar product to what is existing.
I'd then suggest purchasing Rocket Timber Sanding Sheets as there are a variety of grits to choose from as well as some painters tape to mask off and protect the surrounding areas. You should start with the 40 grit to remove the majority of the stain before changing to a finer grit, such as the 120 grit. Once the stain is fully removed from all areas, you could use the product you have gotten to recoat the area. Make sure to use that masking tape to protect the surrounding surfaces and keep a bucket of water and a rag nearby for any clean up.
Let me know if you have any further questions.
Jacob
