A centre line uses a long-dash-short-dash pattern to mark the axis of symmetry of a feature: the centre of a hole, the centre of a round shaft, the centre line of a gate face, the axis of a swing-gate hinge.
Centre lines serve two jobs at once. They tell the reader the feature is symmetric (so you don't need to dimension both halves) and they give a clean datum to dimension from. A bolt-hole pattern dimensioned from a centre line is much easier to read and inspect than the same pattern dimensioned from the part edge.
Drawn at the same line weight as extension lines, lighter than visible-edge lines, in the standard AS 1100 line types.