Interesting report, and a lot of damage for no noticable change in engine indications.
But note the aircraft's track through the satellite picture of the ash-cloud location.....right through the red.
I'm picking there are a plethora of satellite pictures available of the current ash cloud over the south Tasman, and the operating airlines are carefully choosing routes and altitudes to avoid the indicated areas. I doubt pilot observation is the sole criteria for selecting a clear route.
I flew into Auckland from an Asian port on Monday morning, and ATC were being incredibly accommodating and helpful to aircraft's requests for routing/altitudes. I have absolutely no doubt that airlines like Air NZ are carefully checking the conditions of their aircraft's engines through borescope etc and equally no doubt that they would cease operating if any risk of large engineering costs was present.
I guess time will prove or disprove the wisdom of operating or not operating in the current environment. Someone will end up looking very clever, and someone will end up looking very foolish......time will decide who.