Hi @capelli,
A warm welcome to the Bunnings Workshop community and thank you for your question.
It is difficult to say without seeing the area that you are referring to. If you can upload some images, then I'd be able to make a much more educated determination. Let me know if you need a hand uploading images, I'm happy to help if required.
I imagine painting the rest of the area with bitumen membrane won't have a negative impact and will add an additional layer of protection. If it does not detract from the aesthetics of the area and you have additional bitumen membrane, it might be worthwhile to widen the area that you have waterproofed, but without seeing it, it's impossible to say whether you need to or not.
Once you've uploaded some images, I'd be happy to advise further.
Jacob
