I am quite sure this brooch is not the same as the Countess of Wessex's wedding tiara (even considering it could be mounted in a different setting than today).
My impression is that the "tiara" worn in Melbourne in 1901 by Queen Mary (then Duchess of Cornwall and of York) is not a "real" tiara, but a head ornament made from various elements assembled together:
- the two lateral "snail-like" scrolling elements look strikingly similar on this picture of Queen Mary in Melbourne to the ones visible on this other picture of Queen Mary - with a different central element:
- the central element on the picture of Queen Mary in Melbourne is probably one one the diamond brooches in the shape of an heraldic Yorkshire rose that Queen Mary received as wedding gifts - see http://members2.boardhost.com/royal-jewels/msg/archive/1413549745.html Camilla, Duchess of Cornwall, occasionally wore one of these brooches: