Hi @grahamcole,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Being that you are in a bathroom and more specifically in a shower, it is important to note that any holes in the wall will penetrate the waterproofing membrane, which can lead to leaks, and in turn, things like rot, rust or structural damage. It is important that you are aware of this when considering something like this.
If we can locate a stud, this would still be ideal, but you may need to try something like this Trojan Magnetic Stud Finder, which uses magnetism to locate the screws behind the tiles rather than picking up changes in density like an electric stud finder.
If you are still unable to locate a stud, you would want to use nylon hollow wall anchors like these Ramset 6 x 38mm Universal Anchors.
You would also want to ensure that the penetration through the wall is sealed with silicone to prevent any leaks.
Let me know if you have any further questions.
Jacob
