What makes this impressive is that Ultraman Z is one of the few tokusatsu TV shows that won under this category. Past winners include: Ultraman Tiga, Kamen Rider Kuuga, and Tokusou Sentai Dekaranger. The Seiun Award awards the best sci-fi works and achievements of the previous year. It is similar to another sci-fi award, the Hugo Award in which the attendees of each respective convention vote on the winners.
The voting for this year's Seuin Award ran from April 24th to June 15th, with the award ceremony happening on August 21st at the 60th Japan Science Fiction Convention.
[via Anime News Network, for the list of past winners: Wikipedia]