The authority to build a large magazine on Agars Island was given on the 10th March 1871. Work on the site commenced on 21st March 1872 and was not completed until 21st November 1878. The total cost of the construction was £10,154.
The buildings on the Island included a brick and concrete magazine, lined with wood. This magazine could hold 6,980 barrels of gunpowder. There was a shifting lobby adjacent to the magazine. The magazine itself was underground and of cut and cover construction. A earth blast wall surrounded the whole complex.
There was a barrack building for twenty men, a guard room for four men, a foreman and labourers quarters.
The site is now private property but the main magazine would appear to have survived as does the barrack block. A large residential home has now been built on the Island.