it depends what you mean by secluded? There are some relatively quiet beaches within the Sydney metro area, especially if you go mid-week. (A couple are quieter because they're nudist beaches). Even some of the beaches like Bondi have stretches where you don't have to be cheek-by-jowl. But for real seclusion, there are some great beaches in the national parks to the north and south of Sydney. They're secluded because it takes some effort to get to them, a walk of a km or 2. Those beaches are an hour to 90 minutes from the CBD.
I've created a map - not comprehensive, just off the top of my head. I've made it public so feel free to add your own quiet spots.
http://maps.google.com.au/maps/ms?hl=en&ie=UTF8&msa=0&ll=-33.827646,151.285858&spn=0.110517,0.173035&t=h&z=13&msid=116803066728796002745.000496b2a498ba5554a87 about 10 years ago
Good answers. Also try Bungan Beach, which is between two better-known beaches, Mona Vale and Newport beach, on Sydney's Northern Beaches. It's a steep walk down a sealed track at either the North or South ends of the beach, and there's no store, both of which keep the crowds away. The beach is only patrolled at the north end so the south end is usually empty and private. It's about 45mins from central Sydney by car or 1:20 by bus. about 10 years ago