AH,
Understand completely, I'm with you, wouldn't be going after dark.
Basically the plume is over the West to East coast and soon to extend up to the far north if the predictions are accurate. Therefore there is no lateral deviation available such as there may be up in Indonesia with a localised erruption for example. So the only option is to fly under it.
And how do you guarantee avoiding it based on a prediction that has not been entirely accurate throughout the last 24hrs, after dark or in IMC?
Looks like Air NZ is still operating with their amended flightplan tracking after dark......