You can LOOP this by using the FERRY!! A ferry goes from Russel to Opua, and you can continue from there back to Whangarei or elsewhere. Road is really fun now that its not gravel!
As always in rural NZ the surface is a constant surprise. This route is only recently tar sealed and (on this occasion) still lots of gravel patches but they should clear quickly.
Outside of the holiday season I'd expect little traffic except locals so you'd need to be on the lookout. There was also a stock drover with a small mob of cattle.