Julian Rocks - interesting info and good local id's.
Diving the Gold Coast - informative site with good critters list
Australian Museum Fish Site -- lots of id stuff.
Dave Harasti's Underwater Photo Gallery -- excellent resource of images and descriptions, huge concentration on Nelson Bay species. Archive of marine species magazine articles written by Dave. Also includes links to a Marine Species Database located in USA.
Fish Base -- virtually every species of fish on earth. Search by family, genus, species. Pictures and video clips.
Sea Slug Forum -- a site where you can ask questions and post information on sea slugs including nudibranchs, bubble-shells, sea hares and side-gilled slugs. Includes species lists, pictures, etc.
Australian Institute of Marine Science
Australian Marine Conservation Society
Marine & Coastal Community Network
Marine Education Society of Australasia -- their "Marine Links" sections is extensive.
National Marine Science Centre
National Parks Association of NSW -- NPA is a non-government conservation organisation, this link jumps to their marine section
NSW National Parks & Wildlife Service
Reef Watch -- get involved, counting and identifying fish around the world. Subsidised travel available to qualified divers.
Underwater Volunteers NSW - umbrella organisation for underwater volunteer groups in NSW
Byron Bay Underwater Research Group (BURG)
Great Lakes Underwater Group (GLUG)
Nature Coast Marine Group (NCMG)
Port Macquarie Underwater Research Group (PURG)
Underwater Research Group Dive Club -- Sydney (URG)
Terrigal Underwater Group (TUG) Central Coast NSW
Combined Hunter Underwater Group (CHUG) Central Coast NSW
Jervis Bay Dive Club - Jervis Bay Marine Park