My favourite NSW day walk is The Castle.
The best day walk that I have ever done is The Castle in NSW and I consider it a better day walk that the world renown Tongariro Crossing in NZ.
The Castle is located in the Morton National Park south of Sydney NSW, the walk has just about everything in it, with some easy walking at the start/finish, creek crossing, some very steep climbs (walking), some basic rock climbing, need to handle heights and some stunning views.
Tony

Some easy walking at start and end

Scrambling through the tunnel

Climbing lower down

last climbing to the top Plateau

View from Southern tip of The Castle

The trip down, with big drop off to the right.