The reg tweak in this thread fixed my issue.
Thanks to NewMagic (Mark) and those that contributed to nutting out this issue.
A couple of variations worth noting:
1) I'm 8km north of Melbourne CBD - hardly regional Australia
2) I'm running Windows 7
3) I had issues scanning "Prime" channels (9 Digital, Nine HD and Go!) and all SBS channels in MediaPortal 1.1 on a newly built HTPC. They scanned fine in WinTV 7.
I used ScanChannelsBDA to confirm that the problem channels were offset (eg 9 was on 191625 i believe), and that the signal and quality were strong.
After a lot of stumbling around the net and colourful language I happened upon this thread, tried Mark's reg patch (with the latest drivers mind you), and hey presto, all seems now to be working well (touch wood).
Anyway, thought I'd just note my experience for anyone else that has spent ages stumbling around looking for a solution to this.
Now, to get my EPG sorted out...
