Yes this looks like a juvenile eastern brown snake, still lethal even at this size. From memory I think there are a couple of other small snakes in the ACT that have black on their heads (when I last saw them they were in the genus Unechis but now in the genus Parasuta) but I think the black area pattern on the back of the head is always unbroken.
Yes the main one is Suta flagellum (little Whip Snake) with an unbroken patch. It is a grassland species and only known from three sites that I am aware of. Dwyer's snake (another Suta) is similar but generally much darker in body colour. There is the Red-napped Snake north of Canberra that has a double patch. Mustard-bellied Snake to the east of Canberra also has a dark single patch on its head but body colour is much darker.
thanks Geoff and Ken for the comments, very interesting. This is maybe only the third or fourth of these juveniles that I have seen around here. This season appears to be a big one for Red-bellies, such lovely snakes.