Hi Shae, cross-posting my reply to your email here for the broader data community’s info. If someone has more ideas, please jump in!
I also just noticed that you listed the regions slightly differently here than in your email, but it’s still good to double check which regions you need.
As the technical definitions of many boundary types are set independently of other types (e.g. localities vs Local Government Areas), it’s unfortunately very unlikely that there exists a spreadsheet combining these, at least from a government source. For example, Bannister is mostly in the Shire of Boddington, but part of it is in the Shire of Wandering.
Since you’re zooming out quite far, it’s probably good enough that post code boundaries generally coincide with locality boundaries, and locality boundaries mostly line up with LGA boundaries. Depending on the regions you’re referring to these may be defined by LGA, but you’ll need to check the relevant legislation/definitions. However, you’ll need to link this data together in order to find post codes by region.
To start off, you’ll need to determine how the regions are defined. I have only come across regions where the Gascoyne and Mid West are separate, so unfortunately I’m not sure which type of regions you may be using. As an example though, I once had occasion to compare the regions described in the Planning and Development Act 2005 to the regions in the Regional Development Commission Act 1993 (also available as a spatial dataset). The acts define the non-metro regions by listing local governments, with the difference I found being that Serpentine-Jarrahdale was metro in one and Peel region in the other. If you know the legislation that governs your regions, you can look up the relevant Act on legislation.wa.gov.au.
I expect you’re aware that Australia Post provides postcode data including locality. So the other hurdle is determining which localities are in which LGAs for the entire state. For a government data source this would be the Localities and Local Government Areas (LGA) Boundaries datasets made available by Landgate. However these are both spatial datasets, and as mentioned they are independent of each other. If you’re experienced with GIS software, you could overlay these datasets and run an analysis to pull out which locality boundaries fall within which LGA boundaries (or if your regions of interest have spatial data available, you could directly compare it with localities). Apart from that though, you might be able to find information online where someone has already done similar work, but unfortunately I’m not aware of any government sources that would have this information in a table format.