Justin - if you are in Sydney, I have found even as close as Newtown I have difficulty getting Channel 7 (and SBS) as their signals are weak. What you are seeing suggests that scan is unable to lock on to those signals. In Kaffine, when you should be able to see the signal strength for Channel 7 as it scans past it. If it is significantly lower than the other channels then that could be the reason. I have found that my DVB-T card is pretty signal insensitive (Nova T 500), and I needed to get a signal booster and switch on the LNA in the card to reliably get 7 and SBS.
You didn't mention whether you restarted VDR or at least reloaded the router after uploading. Follow niteman's instructions to do that. Also, I have noticed several times that uploading a channels.conf file as you detailed doesn't work the first time. If that happens to me, I simply reupload it again, restart VDR and that usually fixes it. Perhaps just reload the router anyway.