Many businesses are now using their third or fourth generation of computers. What did you do with the old ones? Sometimes they can be recycled as a firewall, print server or even for that new staff member too timid to complain about using a 386 with Windows 3.1. It’s useless old junk, really, but it's hard to dispose of, and some councils will not accept it in hard-waste collections.
See http://www.zerowaste.sa.gov.au/e-waste/ongoing-e-waste-recycling-options-in-south-australia for a list of electronic waste drop-off centres in South Australia.
Dell also has a number of recycling services.
See also http://www.recyclingnearyou.com.au.
Be sure that hard disks are properly erased before disposal: Erasing Hard Disk Drives.