You can tell it's a Tiger Snake (Notechis scutatus) by the distinctive frontal scale, which is a large, squarish scale on the center of its head, roughly as wide as it is long. The face also looks like a Tiger Snake if you're very familiar with them.
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.
