However, with decimalisation, it became 140 km. This has been my reference point for a number of years.
However, the Westgate Fwy was built some years ago and more recently, the Geelong by-pass has been opened, and these may have affected things. It seems as if VicRoads has been updating matters, because just down the road from the 140 km marker, there are two markers that appear to designate a distance of 139 km. One states “139H” and a couple of hundred metres away, the other states “139D”. I haven't worked out the significance of these markers and a search on the VicRoads website didn't turn up anything (but perhaps I missed it). Interesting, the more prominent markers found every 5 km on some highways seem to be few and far between on the Great Ocean Road (edit - where they exist, the 5 km posts mark the distance between major towns, so their location bears no relation to the distances to Melbourne).