I think it's a dubois sea snake, Aipysurus duboisii. Not sure though (not a species option when up loading) if it is, ALA has Coffs Harbour as the southern most recorded sighting for the species. Port Macquarie, Lighthouse Beach.. Sadly it was dead.
This is an Elegant Sea Snake (Hydrophis elegans), characterised by a greatly elongated body and deep and compressed hind part in adults. The pattern and colouration on this specimen is also consistent with this species. Whereas, Aipysurus duboisii is usually dark purplish-brown above with a reticulated pattern over the dorsal surface.