I think the airlines (well both majors in Australia) are in the processes of reducing the main types in their fleet. Economies of scale and all that type of stuff. I think shortly you might see one type between Virgin and Tiger, and I hear rumblings that Q is going to make an announcement over the next few months about the replacement for the 738 that could ruffle a few short haul feathers!
There seems to be a lot of consolidation going on while the going is good....the low fuel prices won't last forever.