I have seen a very good photo of Queen Elizabeth wearing II wearing the plunket tiara (said to be borrowed from the Plunket family for ...) with a very good close-up (I will try to find the name of the book at the end of the week).
Are we sure that the tiara worn at this event was borrowed from the Family. Indeed I am sure that the tiara has bossoms with a central sapphire. Therefore, I think that it could be one of the two others diamonds and sapphires tiaras owned by Queen Elizabeth II (and said to have never been worn)