From my smartphone GPS measurements over a number of days, I am quite certain that Bing imagery for Merredin should be offset 2 metres to the south. But I realised it's too big a job for me to correct such a large portion of the map single-handedly. Map accuracy to 2 metres is still good, so it seems best that I just continue tracing Bing imagery without any offset to avoid creating inconsistencies. Perhaps a mapper with more experience, or better tools in the future, will be able to offset the entire area?

https://www.openstreetmap.org/changeset/146559341