Hi @cheyenneautumn,
A warm welcome to the Bunnings Workshop community and thank you for your question.
As you are planning on waterproofing around your bath, it would be worthwhile to review the NCC Part 10.2 Wet area waterproofing which is the main source of information on modern waterproofing standards.
Unfortunately, looking at Part 10.2.20 Baths and spas, I'm not confident the previous installer has met current standards, as the bath would need to be recessed into the wall. If you'd like to bring it up to modern standards, this might not be as simple a project as expected.
As this is not a newly built home, the choice is yours whether you'd like to bring it up to modern standards, but if you are planning to sell or want peace of mind that your asset is protected, it would certainly be worthwhile.
If you'd like to bring it up to modern standards, I'd suggest contacting a waterproofer for their assistance. They may know a method of waterproofing this wall that would be sufficient.
With this being said, to prepare the wall for waterproofing, I'd suggest rendering the wall with Dunlop Multipurpose Acrylic Render. This is a base coat render, so it is not a super smooth finish, but it will give the membrane a much more even surface to be painted on. Check out How To Render for some guidance on the process.
Once the render has had sufficient time to cure, your membrane can be applied over the render and tiles can be applied over your membrane.
Let me know if you have any further questions.
Jacob
