If you look at the timing of trans Tasman flying, it really stands out that having crews based in NZ works out far cheaper than in Oz (and that's not taking into account the pay scales). Each aircraft would depart an NZ port around 0630 and return approx 7 hours later. Then a second crew would repeat the cycle getting back to NZ by about 2200. The aircraft would remain overnight before doing another four sectors the next day with a utilisation exceeding 12 hours per day. For NZ crews it is simply 2 sector day trips but OZ based crews would be required to be accomodated and fed in NZ - thus much higher costs.
As TT is Virgins Low Cost arm, there's no way they will saddle themselves with the cost of using Oz based crews on trans Tasman routes, they simply won't even fly the route if they can't base crews in NZ.