A snake with a white band around its neck is called a crowned snake.
Without knowing more such as where is it and other descriptions, my guess is that its a white crowned snake. Taken from Qld gov website: The white-crowned snake is dark grey above, with a light grey belly and a broad white band across neck, sweeping forward around the side of the head and across the snout, creating the 'crown'. The crown surrounds the black scales on the top of the head. It is considered to be one of the most common snakes in Brisbane, even occurring in the inner city. It inhabits compost heaps and damp areas of gardens. A similar species, the dwarf crowned snake, has a thinner white band around the top of the head and is slightly shorter in length (i.e. 0.35m). Average length: 0.4m WARNING: VENOMOUS. Not dangerous to humans