I’ll probably just leave the hole depths as they are, it just means slightly longer posts required or a little more concrete - thinking about it some more I’m not sure it’s worth the time or effort to fill them in to try to save a few bucks.
For those who asked, I’m in Christchurch and I haven’t done an exact measurement for height yet but it is a fairly low deck (hence the need for more posts as I can’t use thicker bearers or joists as they’d make it too high). They’ll easily have more than 1/3 of the post below the ground - more like 2/3!