Probably a Robust Striped skink as I aw a couple of more easily identifiable specimens at the same location a week or so later. Not sure because its tail seems very long in proportion to its body size.
I don’t think it’s a water skink. The body is too slender and the tail too long for its length. Also it doesn’t have the typical black colouration along its sides. Water skinks were very common where I used to live in Sydney and I’m pretty sure I could identify one fairly easily.