Horsman will be assessed once he arrives in Australia and, although he is fully fit and there should be no problem with him answering his country's call on Saturday, there is still an outside chance Roberts could be involved.
"I'm just extremely happy to be here," said Roberts, who has played for both the Cardiff Blues region and Cardiff Rugby Club this season and represented Wales in the U21 World Cup in Argentina in 2005.
"I'm getting stuck into training and just trying to enjoy myself. It's difficult to believe I'm here really.
"Last week I was in Canberra watching the game on TV in West Rugby Club, with my friends joking about me being called up and the next thing I know the management are handing out kit and giving me a squad number.
"It's pretty unreal, but I just want to do everything I can to help out. I'm not thinking about the outside chance of me being involved on matchday, its just enough for me to be in this environment and to be able to do something towards the national cause."
Scott ROBERTS (CARDIFF BLUES)
DoB: 05.09.84. Ht: 5'10"/1.81m. Wt: 17st 0lbs/108kg
International record: Uncapped