I concede - it does look like a mangrove water snake.
Now the excuse...! I alway forget how snall Grey Herons actually are, and from some quick rough calculations based on the heron biometrics (assuming eye-bill tip length of approx. 140mm) I make the snake to be about 540-570mm in length, give or take, which is far to small for a python, (even a hatchling at 650mm).
I did initially think of MWS but it just seemed to be too big from looking at the pictures (cue chorus of yeah, yeah, chinny reckon!)
I'll wind my neck in.