The most pragmatic proposals I've seen revolve around optimizing for the most common trips, rather than trying to completely eliminate cars. Optimizing for the easiest 80%, if you will.
For most households, the easiest 80% to mode-change would look something like: - schools within walking distance - shops/cafes/restaurants within walking distance (either by removing euclidean zoning, or creating new, small retail zones) - better public transport specifically targeting the most common destinations (e.g. a central business district) - improving walking/cycling infrastructure so people aren't risking their lives when doing shorter trips
People are still going to want/need cars to go to the beach, home improvement store, etc, but that's fine, because these make up a minority of total trips taken.
Auckland is very suburban outside the center, but the older and newer suburbs have mixed use zoning which manages to support a few cafes, restaurants, shops per suburb. The middle-age suburbs are the worst for walkability. The population of NZ is increasing, and most of the cities are expected to increase in density, rather than size, which should mean more retail can be supported within a shorter distance to everyone. It remains to be seen enough mixed-use/retail zoning gets allocated in existing suburbs though.
I'm not sure how this would play out if the population was stable though. Possibly then like belval says, it would take 50-100+ years as housing stock gets rebuilt in different configurations.