Hello @carmelap
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your shower head.
It appears that the brass pieces have custom sizes and are non-standard. I propose removing the brass flange so that you can get access to the standard brass fitting behind the wall. If the fitting behind the wall is standard it should be 15mm in diameter. I recommend using a standard Shower Accessory Wall Plate Tube & Arm 45 Degree. However, if the fitting is non-standard, I recommend engaging the services of a registered plumber and having them fit in your new shower head.
If you have any other questions we can help with, please let us know.
Eric
