Looks more like and more likely to be a Red-bellied Black Snake, due to habitat and the fact that it was basking in the sun. How long do you estimate it was?
I could not see any red on it’s lower scales, as is usual for Red Belly Black snakes, which is why I opted for the Cryptophis ID. The length is estimated to be around 1 metre.
The head shape with what appears to be a distinct canthus rostralis and a relatively large eye, suggests a Red-bellied Black Snake rather than a Small-eyed Snake. The reason that you didn't see any red is because it may have been in pre-slough, as I can just barely make out what appears to be a milky eye. It also seems to be flattened out, which is more typical of a basking RBB or the defensive posture more characteristic of that species. The fact that it's diurnally active further suggests a Red-bellied Black Snake, as opposed to the essentially nocturnal Small-eyed Snake. However because the image quality is not good, a positive ID is difficult.