Queensland State Archives posted a photo:
Fort Lytton was built in 1880-1882 in response to a fear that a foreign colonial power might launch a naval attack on Brisbane or its port, both of which were located along the banks of the Brisbane River. The fort was designed by Lieutenant-Colonel Peter Scratchley. It was located at the mouth of the Brisbane River, and was designed to deny enemy vessels access to the river.
This was initially achieved by a remote-controlled minefield across the mouth of the river and four muzzle-loading heavy guns. The minefield was closed in 1908, but the guns continued in operation until 1938.
Description souce:
Wikipedia
View the original record at the Queensland State Archives:
Item ID 2692229