Although the Avoca River has a substantial catchment area, the fifth-largest in Victoria, most of that area is on the northern plains where rainfall averages only about per year, and where there is little runoff because the terrain is very flat. The mean annual runoff of per annum accounts for only 0.67% of Victoria's runoff. Most of the water flowing in the Avoca River originates in the narrow upper portion of the catchment area, where rainfall averages about per year, most of it falling in the winter and spring.