McCallum’s Creek, like nearby Bet Bet Creek and the Avoca River (which is next across to the west) mostly does not flow through the warmer months and even in winter seldom carries a great deal of water. Like the two other streams mentioned, it has a large catchment relative to its streamflow.
The surrounding country is north of the Divide and does not get a lot of rain; what there is mostly soaks into the hungry soils and feeds an active groundwater system: the creeks flow only gently.
But there is a threshold point: once the soil is soaked, all further rain in the catchment flows directly into the creeks, and because the creeks are small and far apart, they suddenly morph into vast streams carrying several years worth of water in just two or three days. The flood seen above is almost over: twelve hours earlier, it was storming over the top of the bridge I stood on to take the picture.