ACT it would be stormwater most likely as it is only 600mm down and concrete pipe (clay or terracotta for sewer in 1972 and PVC as it came in) and usually, but not always, deeper (site dependent). If you dig along a little in either direction and find a join you will find that it is just a lip and has no collar and no rubber ring - the absence of which mean stormwater. And the concrete stormwater pipes were often only 2ft (~600mm) long). Although there were some concrete stormwater pipes with collars - but not with full seals -usually just mortar.
It was not uncommon for the stormwater to run in the same general direction and almost on top of the sewer if the mains connection for each were in the same general area - often one side of fence has a stormwater easement and the other a sewer easement. Depending on the layout the lines could cross over.
You can run some water into a downpipe that you think would connect to that pipe you have exposed - it should be easy to hear the water flowing.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.