Why this is happening? Beats me.
My guess is that MEL-AKL is a mug's market compared to SYD-AKL. QF don't have enough new 738s to serve SYD-AKL along with MEL-AKL let alone BNE-AKL. I think it's also reflective of the relative capacity as well (since you get more pax onto a 738 compared to a 734).
I don't usually mind the 734s, though. They are hardy planes and yes you can clearly see the age, but as long as I can get row 4 in Y or an upgrade to a Millennium seat in J, I'm alright for 3 hours. I haven't tried the new 738s with new seats yet.
When will we see more 738s? I doubt in the near future. Trans-Tasman market is becoming more of a mugs game and with NZ shifting to Seats to Suit (because "the leisure market is increasing"), thus deepening the chasm of LCC-ness, unless there is a strategic shake-up on the market or a sharp incident in the Jetconnect fleet I don't think QF will be scrambling quickly to get the next set of 738s for this route.
When it comes down to it, if you want a newer plane with J to cross the Tasman from MEL direct, your only choices really are NZ (the long haul tag flight) or EK.