I did a search on
www.yellowpages.com.au and found the following:
GNC Live Well
Shop B Hunter Connection Sydney NSW 2000
ph: (02) 9221 4250
GNC Live Well
Shop 2 309 George St Sydney NSW 2000
ph: (02) 9279 1770
GNC Livewell
175 Shop G2B Pitt St Sydney NSW 2000
ph: (02) 9230 0855
Go Vita Discount Vitamins
Market City, Level 1 Hay Street Haymarket NSW 2000
ph: (02) 9281 8118
Health & Fortune Pty Ltd
Upper Ground Flr, Ste 6, 370 Pitt St Sydney NSW 2000
ph: (02) 9264 4100
Health Harbour
111 Harbourside Shopping Centr Darling Harbour NSW 2000
ph: (02) 9281 4077
Healthy Life
Westfield Centrepoint Pitt St Sydney NSW 2000
ph: (02) 9232 6718
Healthy, Wealthy, and Young
Shp 6 370 Pitt St Sydney NSW 2000
ph: (02) 9268 0264
Healthzone
Shop 9.16 World Square, 644 George Street Sydney NSW 2000
ph: (02) 9261 4207
Pure Health
Shop T8, Met Centre 273 George St Sydney NSW 2000
ph: (02) 9247 8889
Pure Juice & Soup Bar
Shop G2,310 George Str,Hunter Connections Sydney NSW 2000
ph: (02) 9232 2693
QVB Health Foods
Shop 66, QVB Walk Queen Victoria Bldg Sydney NSW 2000
ph: (02) 9264 3825
Strand Health
Shop 27 The Rocks Centre The Rocks NSW 2000
ph: (02) 9252 2558
QVB Health Foods and Healthzone are the closest to Sydney Central Hostel.
Most big supermarkets (like the Woolworths near Town Hall station) will also have a health food section although this won't be as extensive as a specialist shop.
I imagine that the food labeling regulations are similar to the US. The regulations are the same in Australia and New Zealand. You can find more information here:
www.foodstandards.gov.au