There are two national park areas in Sydney that are perfect for hiking... Sydney Harbour National Park and Royal National Park. As a disclaimer, I'm from www.royalcoastwalks.com.au and we provide guided hikes in Royal National Park.
Sydney Harbour National Park is divided into small pockets of bushland hugging Sydney's harbour. It's a great experience to walk amongst gum trees with city skylines in the near distance. The best starting point is a ferry from Circular Quay to either Manly or Taronga Zoo. See walkingcoastalsydney.com.au for free trail notes and maps of the many walks in this area.
Our favourite is obviously Royal National Park - Australia's oldest park (second oldest in the world after Yellowstone). There are more than 150km of walking trails with the highlights being any section of the Coast Track. Plenty of trails to choose from if you have a car but if you don't, catch the train to Cronulla then get the ferry to Bundeena to start on the Coast Track. See wildwalks.com.au for free trail notes.
Note, the popular (and very beautiful) hiking destination of Blue Mountains is not actually in Sydney. It's very accessible by car and public transport, however you'll spend at least two hours getting there, plus another two getting back. I recommend this if you've got a few days to spend hiking in the one area.
You could also try Kuringai or Lane Cove National Parks. See environment.nsw.gov.au/nationalparks for more information. Have fun! over 6 years agoAnswered by Tara Wells via WorldNomads.com