Depends on which version of NetBSD you're currently running.
I used ns-flash on my NetBSD 3.1 system but it crashed,
but I kinda expected that behavior since 
the information on the following link
doesn't say that it works on 3.1:
http://pkgsrc.se/multimedia/ns-flash

If you're running one of the platforms
listed on that webpage, I 'think' you should
be able to use ns-flash, I don't know for certain,
though.

Hopefully someone else is better suited to answer
this and will provide further information.