 It was bounded by the part of the Hawkesbury River to the south, the Macdonald River to the south-west, and the Hunter River to the north.Northumberland County was named after the English Northumberland, and named by Lieutenant Charles Menzies, commandant at Newcastle, about 1804.In 1852 it had an area of and population of 15,207, and was described as being long and wide and the main coal region of the colony.