Sydney's Royal Botanic Gardens offers a plant identification service over the counter, by fax or mail. Fees apply.
The Royal Botanic Gardens maintains a free online plant database which has considerable information on NSW plants.
The most comprehensive reference book for NSW plants is the four-volume guidebook, Flora of New South Wales. It can be purchased from Royal Botanic Gardens shop.
Plants in your area
If you need information on locally native plants in your area, contact your local council. They will have lists of species and some offer seedlings for growing native gardens.