... the extra bit from Myrtleford to Whitfield ....... I presume a large part of that is stil unmade.
This route is all sealed:
https://goo.gl/maps/NCNvxJxG2vS2 . The section b/t Whitfield and Oxley is generally straight and flat, except for a series of very fast sweeping curves a few km north of Edi, good for 100km/h

+ (so I've been told) make it very exciting. The section b/t Oxley and Myrtleford is known as the Snow Road, seasonally (snow) very busy, heavily policed.
This route via the Rose River Road, has a section between Dandongadale and Cheshunt of good gravel, is acceptable for a road bike:
https://goo.gl/maps/jyZS7F32oxG2 . At least is was a few years ago when I took the Mighty CB1300, trailer and pillion across there (pillion wasn't too happy). It's an important local road, no reason think it would be any worse, it's more likely the local Shire have extended the seal a km or two and keep it well graded..