Of course it is a contemporary design, and a modern tiara.
Maybe you are confusing 'abstract' with 'modern' here...?
The tiara certainly has the shape of a stylized wreath, just like the Queen Désirée ruby tiara is basically a wreath, and both are in the end 'inspired' by real wreaths worn in ancient cultures. Thus the tiara's theme is classical - the tiara itself, just like the infamous 'poppies', is not.
If you look closely, it's also far from 'simple' and not symmetrical at all - thogh it's beautiful how it achieves an illusion of symmetry from afar.
Representing the Australia's national flower, the Golden Wattle, in a coloring as if seen at night (in itself an inspired idea), the tiara and its matching pieces are not of 'neo-classical' design, and in view of all technical aspects - the kind of material mix used, the technics of processing the materials and setting the stones, the whole way the design is executed - it should absolutely be labeled 'modern' or 'contemporary'.
For 'neo-classical' jewels, motifs and settings, see for example the Norwegian amethyst tiara, Queen Elizabeth II's aquamarine tiara, or Queen Silvia's neo-classical diamond fringe tiara/necklace.
Hey I love the new tiara and, truth be told, I am somewhat perversely intrigued with the idea of seeing her wear the poppies. 
