One of the traditions of the commemorations for ANZAC Day is that each country (Australia & New Zealand) takes turns to lead the ceremonies.
This year New Zealand did the honours.
And so, the Duchess wore this year's New Zealand version of the poppy.
Even though this is a highly stylised poppy, it is meant to represent the same poppy as used in the commemorations of many countries -- the red poppy which sprung up in "Flanders Fields" as the Canadian poem says.
I have seen references to the Duchess' brooch being meant to represent maidenhair fern and that this is a some sort of link or mark of respect to the Kiwis.
I doubt if any Kiwi seeing the Duchess' brooch would have any such thought. Maidenhair is native to many countries (including New Zealand & Australia) but I doubt if anyone associates it as a national symbol or even plant.
I think this is one occasion where imaginitus has taken over.
When the Duchess was in Australia in 2014 for ANZAC Day she added a symbol which is used by many Australians -- a sprig of rosemary.
Rosemary was used traditionally in Australia because it grows wild on the Gallipoli Peninsula. The symbolism of remembrance is also important
When I was young the poppy was not used in my part of Australia on ANZAC Day --only a sprig of rosemary. I say in my part of Australia because I later discovered it was used in Adelaide. Poppies were used for Remembrance Day. Now poppies are used for both days.
For the day time ceremony in 2014 the Duchess wore both a poppy & a sprig of rosemary. Duke wore only the sprig of rosemary.
For the Dawn ceremony (the most important one) they wore only a sprig of rosemary.
Many today use both rosemary & a poppy.
In Australia rosemary is still the choice of many, especially the old diggers (soldiers).
The current Prime Minister wearing rosemary
So the style of poppy worn by the Duchess of Cambridge on 25 April 2022 has no special meaning, apart from the fact it was the version used by New Zealand this year. The fact that the brooch might represent maidenhair has no significance to either New Zealanders or Australians. I cannot comprehend why anyone would think it would.
The poppies & pins/badges representing sprigs of rosemary are sold in the lead up to the day & on the day. All the money raised is for charities dedicated to the families of veterans.
Because many also have no idea what the New Zealand Silver Fern is, below is a photo. It is not a dainty little fern. It is a large tree fern which grows well over 10 metres (or 33 ft).
I suspect that the Queen's NZ silver fern brooch has misled many.