We’ve had a few threads asking this question over the years and they’re very parochial, with answers reflecting where someone lives rather than a solid knowledge of training across Sydney. The capacity to WFH on at least some days is likely to influence your location preference.
Much depends on how you want to structure your training. If you’re happy to do all your midweek bike miles indoors vs a choice of beautiful climbs on quiet roads through national parks. Endless miles of running trails vs running around the city vs along the beaches. There will pretty much be easy access to 50 metre pools wherever you decide, but closer to beaches offer OWS year round.
Eastern suburbs/inner city/inner west.
Swimming pros: access to ocean swimming at Bondi, Coogee, Clovelly, Maroubra, etc.
Cycling cons: Loads of traffic in this area with Centennial Park (flat 3.8km loop) being the only safe option that isn’t constantly stop-start. Unsuitable for long rides locally. If you live more than 5km from Centennial Park you may even prefer to drive there from home.
Running: Botanical Gardens/Opera House/ Harbour bridge in CBD, but you’ll have access to this on work days wherever you live. Bondi coastal path is beautiful but crazy busy. Centennial Park great for volume miles on soft surface (grass), but
West/South West/ M7 bike path corridor.
Cycling pro: The 60km M7 mixed-use path is the safest cycling in Sydney. Traffic free, well lit, water refilling points and frequented by cyclists, runners and walkers. But flat and boring adjacent to a busy motorway.
Also Parramatta Park for running and cycling, about a 3km loop, If you live relatively close.
While some areas are nice, generally not the most attractive suburbs.
South
A local will undoubtedly reply to disagree with me here, but what was once the training ground of a string of World Champ Aussies (Welch, Jones, Macca, etc) and one of the best triathlete playgrounds in the '80s and '90s, has now been overtaken by the Sydney’s population growth. The best bits are still good, but compromised by access for safe cycling.
The Royal National Park used to offer some of Sydney’s best cycling, but many now regard it as too dangerous with its winding single lane roads with narrow shoulders and speeding traffic.
The Park remains a jewel, with an abundance of spectacular trails for running (home to a 50 miler in the '80s, with no loops or backtracking), from gnarly single track to the shady well-groomed Lady Carrington Drive, following the river for a pleasant 18km return run.
Wherever you settle, make sure you run RNPark’s Otford to Bundeena coastal trail (30km).
Suburban beaches like Cronulla etc are still pretty good.
I’ll return later to review the Northern Beaches and Northern Suburbs.
I lived as a triathlete and cycle commuter in Sydney from 1984 to 2018, training, racing and working over much of Sydney.