I downloaded NetBSD-6.1.5-macppc.iso and burned it with the "disk utitlity" of a mac (have a german version, don't know extactly how the tool is called in english versions) to a cd-rom.
on the system, where i am planning to install the os I typed the following in the openfirmware console:
boot cd:0 NETBSD.MACPPC
I hear the sound of the cd-drive, then i get this:
boot cd:0 NETBSD.MACPPC DISC-LABEL: LOAD (noninterposed) notsupported-loadsize=0 adler32=1
LOAD-SIZE is too small!
Some powerpc macs are quite fussy about the bootloader, and I seem to recall there were some changes which cause issues with older macs.
As a test it might be worth trying a netbsd-5 or netbsd-4 ISO to see if it boots for you