One of the Apple Maps data sources is GeoNames (geonames.org), a free data source available under a Creative Commons Attribution license and also used with the search on OpenStreetMap. If you search for Mildura at OpenStreetMap (openstreetmap.org) you'll get the correct location of the town, but if you scroll down the GeoNames list you'll also be offered "Mildura Shire" as a search option. Click on that, and the pointer moves to a location similar to the Apple Maps screenshot, about 70km away from the actual town. It seems that's the location that Apple's search has chosen to prioritize, over the actual town.
Mildura Shire is listed in GeoNames as a "second-order administrative division", so it's probably similar to the middle of a county or council area.
This isn't a problem with datasets or incorrect data, it's a problem with how Apple is searching & prioritizing the correct data they already have. Choosing the "town" instead of the "second-order administrative division" would be appropriate here.
For what it's worth, GeoNames already prioritizes the town of Mildura correctly in their search engine.