Swimming around mangrove roots searching in crevices and holes
This snake looked dead until I touched it and it moved. The close-up shows an "alive eye".
NSW National Parks has a brochure noting that 11 sea snakes have been observed in NSW but only rarely and as vagrants, the exception being the Yellow-bellied Sea Snake (Pelamis platurus). That one has a different head shape to this record and the head scale pattern does not quite match. So I checked all 10 species on iNatAU and there are few or no records for NSW - Stokes Sea Snake has a record (Coffs Harbour), 4 records for the Bar bellied Sea Snake and 2 for the Olive headed sea snake.
The photos look closest to the Olive Sea Snake (Aipysurus laevis) but the head scale pattern is not quite the same.
Single individual seen on surface, from Exmouth Dive & Whalesharks boat trip. Did not estimate size at time, but fairly large - over 1 metre I recall.