Given the schedules and time zones, I think you'd struggle to get four crossings out of a 737 per day.
Sorry Keg, I disagree.. Most Jitconnect aircraft leave from NZ in the early morning, to arrive into Australia around midday NZ time, then back to NZ around early avo, then turn around and do another Trans-Tasman to be back around midnight NZ time...
i.e AKL-MEL-AKL-BNE-AKL
Personally I believe that the Trans-Tasman is a loss leader, like that $2 bottle of milk at the supermarket.....