Looking at Flightradar 24, the Virgin aircraft made the decision to divert to Mildura first, with the Qantas aircraft about 5 mins behind. Note, the Qantas did a few holding paterns to the east of Adelaide.
The Virgin aircraft disappears of Flightradar first with the Qantas aircraft still in view for another 3-5 mins. Based on this it would appear Virgin arrived in the Mildura area first, but Qantas who were behind them landed first???