When our steel posts are dipped in molten zinc at 420 degrees Celsius, it does more than provide a rust resistant protective layer on the steel, it actually produces a chemical reaction between the steel and the zinc which provides layers with various beneficial properties.
Note that the Delta Layer, which contains from 7 to 12% iron is over 50% harder than the original steel. The outer zinc layer, by comparison is rather soft, being 50% as hard as steel. What this means is that even if the soft zinc outer coating is scratched or chipped off, the steel is still protected considerably more than a painted surface.
And even if a scratch makes it through this hardened Delta layer, the zinc coating acts as a sacrificial anode, which greatly reduces oxidation of any exposed steel. This is why steel ships apply sacrificial zinc anodes to their hulls.
In most Australian conditions, hot dip galvanizing of steel posts should keep rust at bay for over 50 years.