The need for the RAN's AUSCDTONE to destroy a stick of ancient ordnance forced the closure of Sydney Harbour to arriving and departing merchant ships and small craft last month.

Carried out successfully, the demolition provided one surprise.

What was first thought to have been shells from WW11 turned out to be 9 inch shells made around the Crimea War era and used by six guns positioned on South Head and manned by soldiers based at Victoria Barracks.

The then Commanding Officer of AUSCDTONE, LCDR Mike Gough said a civilian diver who "knew what to look for" found three of the four shells lying in 22 metres of water, 200 metres south of the rocks of North Head.

"They were in a line as if dropped off a moving boat," LCDR Gough said.

"The diver informed HMAS Penguin which informed us."

"Our divers did an inspection, confirmed they were ordnance and prepared a plan.

"One was to lift them and take them to the Intercontinental Shelf, another was to detonate them at Jervis Bay and the third to destroy them in situ.

"Other authorities such as the National Parks and Wildlife Service, the Waterways Authority and Police were informed and became involved."

On Tuesday, August 14 Diveboat Seal, accompanied by a RHIB took 12 divers under the command of LCDR Gough and CPO Tim Hayes to the scene.

In relays, eight divers went to the bottom to lay "red cord" (an explosive line) across the three previously identified devices.

With the Sydney National Park at North Head cleared of tourists and with Police and Waterways patrols keeping small craft at least 1600 metres away, the "red cord" was detonated.

"The plan was to clear the barnacles off the shells to identify them," LCDR Gough said.

"We had also laid some cord over what was thought to have been a rock.

"When the cord detonated it split open a fourth device.

"The divers saw it contained black powder and was unstable.

"Other devices had fuses," he said.

LCDR Gough said it was decided to destroy the ordnance in situ.

He said that although the task itself was "what his team does", the exercise was a challenge because it involved a busy port.

The port was closed from 11am to 1.30pm.

By Graham Davis