You all appear to be correct - I was under the impression they used the A320's for the HKG & SIN routes, but checking forward that's not the case. I don't keep records of what i'd flown out of USM (apart from ensure I caught a 717 just before they got shelved!) but you're all right looking forward.
In which case I have to question the belief that I had that PG had given TG two daily rotations under the threat TG made to open another airport on USM. Like the long heralded airport in Koh Phangan, i'll just read that as pie in the sky! The BKK-USM flights on TG always hovered around THB5000 o/w - and loads were seemingly great, but maybe the costs of maintaining their last 737-400 stop this being practical.
Mind you Wellington has a 6,827 ft length runway and takes several A320's daily (that i've checked!). All very odd.