Following World War II, the New South Wales Government Railways embarked on a program of modernisation using aluminium construction skills developed to support wartime demand for aircraft components. The result of the program was a series of diesel multiple units designed to work in pairs, suitable for lightweight branch working, an improvement to the wooden-bodied railmotors.

This design received a revision in 1961, with improved engines and transmissions designed to replace steam locomotive-hauled services in the Hunter Region. The class leader of the revised design, set 621/721, commenced trials on 8 August 1961 and was delivered to Newcastle on 21 August 1961.

The 620/720 railcars were a regular performer on the Hunter Line towards Dungog and Scone, providing faithful service for over 45 years. They also worked routes between Grafton and Casino, Orange to Mudgee via Lithgow, and were used on the Canberra Monaro Express during periods of high demand.

Originally fitted out with a mix of Rolls Royce and Cummins engines, the class received a substantial upgrade in 1984, being fitted out with dual 335-horsepower Cummins engines with new transmissions. These works were highly successful and as a result were also brought across to the larger 900-class DEB railcars.