Qantas does not self-insure its aircraft hulls. For a start it doesn't own all of them. Many are leased and I can assure you that lessors require 'real' insurance.
Lessors even require a right of approval of the insurer - no dodgy insurers allowed. Lease language sometimes allows lessee (airline) a choice, but from only a subset of insurers, and to the satisfaction of lessor.
Self-insurance might be possible for wholly-owned aircraft, but if there is any financing of them I'm sure the financiers would not allow it.