That's a little bit short-sighted considering they will have a life of 30-40 years+. In that time the brown stuff might have hit the fan about oil and the Feds (not Abbott obviously) will be madly electrifying all the interstate freight lines - on AC. The DC obviously won't be extended, except (hopefully) to Nowra.
If the magnitude of the brown stuff flying around is sufficient to make AC traction worthwhile for interstate freight (something which isn't even remotely on the radar because the economics are woeful), then any fleet planning that you did today would be wildly irrelevant.
"We are going to run out of oil - therefore we should use electric trains" isn't long sighted thinking, it is tunnel vision that ignores the wider implications of what running out of oil (to that extent) actually means. The diesel consumption by the interstate rail fleet is hilariously inconsequential in the scheme of oil derived fuel in Australia - if you wanted to reduce oil dependence there would be so many better ways of going about it (and some of those ways might even result in a significant increase in diesel consumption by rail) than by stringing wires over long distance.
If there's going to be incremental extension of the electrified network, then something compatible with the existing DC system is probably the front runner. One day perhaps the capital cost savings associated with the higher voltage system will outweigh the cost of complexity of requiring mixed voltage or split fleets and you'll see high voltage AC used on an extension, but I think that extension distance would have to be over a pretty serious distance for that to be the case.