Welcome to the Community @CatJ
First off the demo mattress has been well used even though they rotate them out every few months so a new one will usually be firmer than the demo - unless you tried the demo shortly after it was rotated into the show room.
Read your rights under the Australian Consumer Law. You can find many posts here offering advice in the how to’s, but your next step would be to write a formal Letter of Complaint. This link is an intro. Basically you were sold an inappropriate product. Your letter needs to reflect appearing before a magistrate who knows nothing, starting with a review of your experience and statement of your rights, what you want, and by when.
and one of the many,
Please let us now how it goes.
