The threat with a low-density cloud is the potential maintenance cost; not crashing. Inspections – possible maintenance – and servicing is needed for aircraft that fly through an actual or forecast ash cloud (low density clouds may not be visible).
Air New Zealand dodged the forecast areas and monitored their aircraft – the Chief Pilot said their was no evidence of ash ingestion.
I was told by a pilot that Air New Zealand was getting better than public information about the position and height of the cloud, and the methods used were reliable – it seems to have worked, and their customers are happy.