Suggest you look at Alan Kohlers report on the Airline duopolpy and see how much money is on the table for profit.
QF and VA have flooded the market with so much capacity there is nothing left to profit from. Any airline, low cost or not will head for the door when they see the money being tipped in to maintain market share. It was openly believed that VA would have hit the wall if it didn't receive the capital raising from its three single biggest shareholders. That money was essential to purchasing a low cost competitor to meet the Jetstar threat at the leisure end of the market.
If another Low cost carrier stepped into the Australian market, which airline do you think they will be cheaper than? Any route they try and establish themselves on will only be met with competition from TT and JQ.
How much lower will their cost be than the established low cost incumbents?