I used to live in Tamworth and was a regular user of the New England Hwy between there and Newcastle.
As Sam said, the majority of traffic on the route is local. Especially between Maitland and Muswellbrook. A lot of people going to from work at the mines and power stations, and to other jobs too.
Back in 2006, the bypass proposals were all about just getting the trucks that are going through to use a bypass. The Muswellbrook one in particular was marketed just as a heavy vehicle bypass and not intended for general traffic.
There was a fair bit agitation to extend the dual carriageway beyond Branxton to Muswellbrook though, as the traffic numbers and conflicting movements suggest it is needed. The latter being more the issue. There’s also been some work on new housing in the region too, which will also add pressures to it.