This issue has been explored. Kelvin Henderson, former Sydney Kings import, lodged proceedings before the Human Rights and Equal Opportunity Commission in 1992. His pleadings were based around racial discrimination. The Commission found that neither the NBL Managment or ABF had based any rules on race, so his case was dismissed. You can read the transcripts here:
http://www.austlii.edu.au/cgi-bin/disp.pl/au/cases/cth/HREOCA/1992/8.html?query=%5e+basketball
Henderson v NBL Management Ltd & the Australian Basketball Federation Inc. [1992] HREOCA 8 (25 May 1992)
Courts are reluctant to interfere with administrative decisions and rules. An example is the AFL Draft. A textbook example of restraint of trade, would be highly illegal in normal business dealings. There has been talk of challenging the Draft system on constitutional grounds for some time, but it would be a mammoth case, and have consequences for all professional sports.
I sat in on the Natalie Avelino case earlier this year, and alot of these topics were discussed, but never determined by the Court..