I don't, but I'd love to see some if anyone has any.
The problem is that because the only interest on the loan is CPI, the HECS system is essentially a fixed pool of money that is temporary loaned to students then paid back, real-dollar for real-dollar. By not paying it back, it's essentially 1 student's volume of study that is lost out of that pool.
I don't know how significant that 'leak rate' is though, so I can't put a figure on whether it's stressing the funds or not.