A large sinkhole has recently appeared on a street in the middle of Melbourne’s CBD.
“It [was] confirmed the issue was caused by a crack in a barrel drain running along the street.”
Ordinarily, “a crack” in a pipe by itself is not enough to cause a sinkhole.
But degredation over time can lead to compounding of pipe defects such as tree roots growing through the cracks, corrosion of any pipe reinforcement, and infiltration/exfiltration (water getting in/out of the pipe).
The biggest culprit here is likely infiltration/exfiltration.
Infiltration/exfiltration can cause the bedding material supporting both the pipe and the road surface to slowly loosen and wash away, which is how a lot of sinkholes develop unnoticed.
See more here.