After seeing that Alison Booth's paper from the Quarterly Journal of Economics was the most downloaded paper from ResearchGate at Crawford this week, I was curious what fraction of employees at Australian universities belonged to the National Tertiary Education Union. Apparently NTEU has 26,000 members. It also seems that there are 113,000 employees in the university sector. On that basis the unionization rate would be only 23%. Of course, quite a lot of those are casuals or PhD students working as lecturer A etc.* But the total number of full-time staff is 86,000. That implies 30%. Also there were 67,933 staff on continuing contracts. If the latter is the real target market for the union then the rate is 38%. Based on this, social custom doesn't work well in the university sector to overcoming free-riding. Let me know if any of my assumptions are wrong as this is the first time I've ever looked at this issue.
* There were 41,730 academics at levels B and above, but the union also represents non-academic staff.