I get asked this question constantly. Here is the legal definition from the legislation:
in relation to a length of road, means an area in which either of the following is present for a distance of at least 500 metres or, if the length of road is shorter than 500 metres, for the whole road —
buildings, not over 100m apart, on land next to the road; and
street lights not over 100m apart.
Be aware that the definition is determined by EITHER of the above conditions, not BOTH.