It sounds like you've got the same configuration I have (single LAN, NetBSD
is the external "router" to the outside world).  All you should need is to
enable GATEWAY in the kernel (or use sysctl -w net.inet.ip.forwarding=1),
and to remove the default -q option given to routed when the system starts
up (look for routed_flags in /etc/netstart).  If you have multiple internal
LANs, then you probably want to pick up gated.