It is not a Golden Crown Snake as the "crown" does not meet in the middle behind the head.
Not all young brown snakes are banded... only some are. All possess a dark blotch on top of the head and a dark band across the neck at hatching. These fade rapidly with growth.
It is not a Small-eyed Snake as those in the Sydney area are dark black and shiny. The head shape and body shape is different. They are also nocturnal and if disturbed from their daytime retreat they are not likely to enter an open area in search of another retreat.
Brown Snakes, Marsh Snakes and White-lipped Snakes will forage in open areas for lizards if they can sense them. They will also stop and utilise warm surfaces in open areas to raise their body temperature if needed.
I believe you can safely rule out Eastern Brown Snake given the head pattern (no dark band across the neck) and the colour of the dead leaves to compare the snake with.
The only two grey snakes in Sydney are the White-lipped (not sure if that extends to Turramurra) and the Marsh Snake. White-lipped Snakes are thinner bodied (not by much) and tend to be grey all over in Sydney.
Dark Marsh Snakes will often have lighter, yellowish colour area around the head. So while I cannot say with certainty, I would be 95%+ confident it is a Marsh Snake. Apart from the two white lines on the face, that Geckoman illustrated, if you saw any of the underneath of the snake (the ventrals) and they were quite dark, that would confirm the identity.
Blue