Found just off O’Hara Is. near Pretty Beach, Murramarang National Park.
The entire plant (or patch of plants) was about 12-15cm across, comprising many narrow, irregular, cylindrical red-brown branches, each maybe 1½ -2cm long. The arresting feature was the flecks of iridescent blue (about 1-2mm wide) scattered seemingly at random on branch ends. The sighting was a solitary occurrence.
Pterocladia sp fits some of the observations, but its branches are flat, not rounded and lack any iridescence.
Apologies for the photo quality – the wave action at ½ m was strong.
I suspect this will need a future specimen to be collected for positive ID, but would very much welcome suggestions.