The internet is littered with the bodies of canyoning web sites, dead and dying. There are only a few that have stood the test of time, though some others survive on the life support provided by the Internet Archive (archive.org). Sites that have not been updated recently are noted below. In a rough order, the sites I think are most worthwhile:

Other useful pages (loosely related to canyoning in the Sydney area):

Weather and rainfall

River heights

General links

This is a random selection of interesting sites with information on topics such as abseiling, techniques, rescues etc. Note that these pages do not teach you how to abseil, how to use the techniques, nor even if the techniques are safe. If you intend to use any of the techniques you should train them in controlled conditions with appropriate safety measures before using them in a canyon. Read the disclaimer

Tape harnesses