It seems a complete lottery in terms of who manages to secure Red Zone for Sydney. My sister and I tried for ~45 mins yesterday, from 3pm on the dot, while it offered Red Zone but then said it was not able to provide the quantity we were ordering, even when we tried to buy one at a time. Then eventually it greyed out the Red Zone option and said the allocation was exhausted.
I decided to just grab general admission instead, which means I at least have a bird in hand. Will try again for Red Zone in the Live Nation presale, but I'm certain the number of people competing in that presale is going to be many times greater.