Standard qantas response hey keg????
The density has absolutely no relevance if you are not in the affected airspace.... The modeling accuracy wrt the altitude of the cloud is quite high hence the risk assessment conducted by every airline that flys into Australia except Qf and tiger, allowed for continued operations.
But hey what would airlines the likes of Cathay, emirates, ethiad, south African, Singapore, Malaysian, united and virgin know.......
An interesting point of note on day 2 of the ash saga.. Was operating a service hba-syd at 180 below the forcast ash area and observed a contrail well above our level. Atc when quieried, advised that it was a Qf 737 operating in the ash advisory area at 320..... I guess qantas must have knowledge the rest of the universe doesn't.......