Gravel riding is usually more informal.
We're still uncovering these events in NZ. You can see any we've found in the calendar.
Auckland Cycling Group on meetup does some.
Dynamo Events has run a NZ Gravel Nationals previously. Maybe it will return.
Craters Classic is an MTB event but apparently is gravelly as well.
Gravel and Tar is a pro level road/gravel event. NZ's version of a cobbled classic?