Queensland State Archives posted a photo:
One of Fort Lytton’s roles was “Inner Inspection Station”. This meant that it could send out inspection parties to board and inspect ships to ensure it was safe to allow them to proceed up the river. A ship (HMAS Kinchela) was permanently anchored near the middle of the river off Fort Lytton.
Two permanent heavy steel boom nets blocked the river permanently on the far side of Kinchela. Two moveable steel boom net gates blocked the river between Kinchela and the fort and were opened and closed by large winches mounted at Fort Lytton and on Kinchela.
Searchlights illuminated the net when required. Warning of vessels approaching the boom was provided by an indicator loop and photo-electric beam systems across the river mouth downstream of the fort. Guns at the fort were used to protect the boom gate.
Description source:
Wikipedia
View the original record at the Queensland