I have personally not redeemed Krisflyer for this route or with Virgin Aust or United.
Here are my 2c:
I would imagine that you would have to find available “reward seats” on Virgin to be redeemable on Krisflyer. Same for United airlines; you have to find available United saver seats redeemable on Krisflyer. If you do not pass the above test, you would most probably not find the seats available on Krisflyer as usually, a partner does not see the same allocation of seats as some FF programs keep some for their own members.
Your most challenging leg would be the AUS-USA leg. That is very very popular so they get picked up very very quickly (sometimes as soon as they are released).