HA! Z fare to SYD, I'd like to see that. They had one last year in the slow season for about $5200 RT midweek, which is the best I have seen to date. It makes a RTW (especially if you can start it in Asia) look especially appealing.

If the lowest fare is the D fare at $6600, then consider a RTW, which even starting form the US, is only $7400 for a (up to) 29k trip. Of course, if you can start in BKK, CMB, et al. then you would be looking at $4500 for a 29k RTW ticket.