It is a bit unusual, but it's become a tradition for more than 40 years now.
Here's a more detailed explanation:
http://nzb.co.nz/info/filly.cfm
I haven't heard anything about The Filly, so I would assume there is nothing serious. The track has ended up being quite dry at Ruakaka today, which might not be suitable for her.
What a win by Moochi Me. So good to see her back.