It had been intended to construct six ships, but the last pair, Polyphemus and Minotaur, were ordered in 1934 as the 9,100-ton Town-class Southampton and Newcastle.

The Arethusas were a smaller version of the Amphion group of the earlier Leander class, having the unit machinery layout and two funnels of the former.

Therefore, no reduction in speed (i.e. machinery) could be accepted, and savings had to be found in armament, size and protection; the Arethusas suppressed 'X' 6-inch gun turret and were 50 feet (15 m) shorter and displaced 1,250 tons less than the Leanders.

Welded construction was widely used for the first time to save weight, over 250 tons being cut off the original specification.

Galatea had extra plating added amidships after completion to reduce wetness and to protect the boats.

She landed her catapult during a refit between October 1940 and January 1941, when she received two quadruple 2-pounders and eight single 20 mm Oerlikons, as well as Type 279 air warning radar added at the mastheads.

The former were removed in the spring of 1942, as were the single 4 in mountings (replaced by twins as per her sisters) and a further four 20 mm Oerlikons added.

Between March and December 1943, while under repair in the United States, the 2 pdr were supplanted by quadruple 40 mm Bofors mountings Mark II, three single Oerlikons by four twin power-operated mounts Mark V and had the radar fit modernised.