Although the Avoca has a substantial 6900 square kilometres catchment area (the fifth largest in Victoria), most of that area is on the northern plains where rainfall averages only about 350 mm per year, and where there is little runoff as the terrain is very flat. Most of the water flowing in the Avoca originates in the narrow upper portion of the catchment area, where rainfall averages about 600 mm per year, most of it falling in the winter and spring.
Of all the Victorian rivers in the Murray-Darling Basin, the Avoca is the most variable. In theory, the average annual flow is 85,000 ml, however recorded actual flows have varied from almost five times the average figure in very wet years to 0.5% of the average in drought years. It is normal for the Avoca to stop flowing for weeks or months at a time during summer and autumn.
Although it is the only river of significance in the area, the Avoca has had no major water storages constructed on it, merely six weirs of only local significance.
It is little used for irrigation as during the peak demand period (summer and autumn) it is often not flowing at all. During low flow periods Avoca River water is usually too saline to water crops with, but can still provide drinking water for sheep and cattle.