A contentious one...
I must admit that when I first looked at the photo I immediately thought “Whip Snake”. It was only after I began examining it more carefully that I realized things were not as they should be for a whip. Under magnification the eye appears as a black circle, lacking the black tail to the comma underneath. The size of the eye is also too small, especially for a half grown animal. On a full-sized whip the eye occupies a good two-thirds of the lateral head profile. What is definitive for me though, is the tail. That tail does NOT belong to a whip snake. It is far too short and tapers far too quickly.
The attributes of the snake do fit that of a juvenile Eastern Brown. They can be a slender build for a large elapid and this specimen is looking skanky and in need of a good feed. The slightly darker blotches on the crown of the head and the nape would correlate with the original juvenile pigment there. It appears to be developing the pale head and neck seen in some adults. As for the area, Collaroy stretches back from the beach a fair way and there used to be a lot of bush at the back of Narrembeen Lakes and either side of the road through there – Terry Hills Rd I think. So you have some extensive bushland not that far away. Just as an aside, a mate and I used to chase Jacky Dragons in the shrubs on Dee Why beach as kids. It could also have been transported in with landscape supplies or firewood or similar by a neighbour.
It is not a Marsh Snake – either colour form, although I have only ever seen the lighter ones with dark heads in Sydney. It lacks the distinctive facial markings and only the head should vary in colour to the rest of the body.
I cannot think of any other potential candidates, so I would reckon it’s a young Eastern Brown Snake.
Blue