I didn't realise that we in Oz were the only ones to refer to them as doonas, and that it is actually a registered trade name. Wikipedia says:

In Australia it is called a "Doona", from a trademarked brand name derived from the Old Norse dunn meaning "down feathers". Though still registered to the Tontine company, the name "Doona" has become a generic term for a duvet or down quilt. Originally the term continental quilt was the standard name used across Australia; some regions of Australia still use this term today.