Hi @lunarchic,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
As I'm sure you're aware, there is a pretty concerning lack of waterproofing behind this bathtub, which I imagine is part of why you are planning to gut the bathroom in the future. Ultimately, the only great way to fix this problem, due to the lack of waterproofing, is to rip out all of the sheeting and start again.
Unfortunately, due to the location of the framing and the location of the tiles that would need to be removed to expose additional framing, you would struggle to patch this gap with a fibre cement sheet product.
If you are just looking to carry out an aesthetic repair to make things look a bit nicer until you can conduct a proper repair, then you could use some expanding foam filler primarily to act as a backing. You can then apply some Sikaflex 11FC to cover the expanding foam and prevent water from getting into the wall cavity.
You could then attach tiles over this to blend everything into a uniform surface.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
