Eastern Snake-Necked Tortoise. The shell is about 20cm long
I know she is female because:
1 - The plastern (under-side shell) was flat. It is concave in males so they can... um... you know... fit together.
2 - She was wandering. Male find a nice water hole and sit. The female has to do all the searching for a mate, then wander looking for a nesting site. Eggs are laid in November and hatch after a couple of months.
These are aquatic tortoises, only coming ashore to lay eggs or to find a new waterhole. This photo was taken using a polarizing filter to kill the reflection so the animal is visible. The water is approx. 3 feet deep, but dropping to 15 feet further out.