The info I can find is that it was a last minute decision to start him at 12 but not that he hadn't been training there.
"The decision to start Hunt against Fiji came at the 11th hour, with Wallabies coach Michael Cheika changing his mind during the Thursday session, knowing full well the 30-year-old had only lined up at inside-centre just once for the Queensland Reds."
Hunt's selection was a surprise - even to him - after spending the Super Rugby season as Queensland's fullback but coach Michael Cheika said his hunger and resilience couldn't be ignored.
"It was a little bit surprising given I've been playing at 15 for the last couple of years but when it comes to Test footy you take whatever jersey is given to you," Hunt said.
"I was a little bit shocked but I've managed to get my head around the plays but I'm looking forward to it and come game day I will be ready."
Taken from two separate articles.
Seems to me like he was destined for a bench role and then Cheika decided to start him. I can't find anything to suggest he was training as a fullback only for the Wallabies. That would seem strange given Folau was healthy.