Black Hawk helicopters in Manly Ferry training exercise

A pair of Black Hawk helicopters has circled and dipped over the Harbour, as part of a military training exercise.

From about 4pm today, a 9NEWS crew captured the counter-terror training exercise over the city.

At one point, the choppers conducted a manoeuvre over a moving Sydney Ferry, which appeared to have no passengers aboard.

It then hovered low to the water, creating a dramatic whirlpool from the downward force of the rotors.

The exercise was part of regular training conducted by Defence, one of several conducted this month in the Hunter and Shoalhaven regions of NSW.