Trouble getting forward zone to work

We've just installed DSfW and set it up with it's own zone (internal.shcs.sa.edu.au)

I wanted to setup a forward zone on our existing name servers used by our clients so they can resolve the new ad zone, but I'm having troubles getting it to work.

I've created the forward zone and assigned it to the existing name servers, I've added the DSfW name server into the forwarding list but I still can't resolve the new ad zone, I always get NXDOMAIN from nslookup when trying either internal.shcs.sa.edu.au or dsfw1.internal.shcs.sa.edu.au.

The 2 name servers are running OES11 and the DSfW server is OES11 SP1, both 64bit.

Thanks
Matt