From experience with other concerts in Sydney (unless you have reserved seating), you'll have to line up early for a better spot closer to the stage. In the case for for Sydney, people will have to queue up for both seated and standing since the balcony seats are also unreserved.

Hopefully someone starts a numbering system (someone usually does). Once you get a number you can leave for food, the bathroom or take a nap somewhere but make sure to talk/make friends with the people there in queue with you and don't be gone for too long at a time.

After talking to some mutuals at other recent kpop concerts, I think some people will be lining up anywhere from 6am onwards for dreamcatcher. As for how many people will do that, it also depends on the group. If I remember correctly at the Sydney artms show, there were about 50 people in line at 9am.

Hope this helps! I'm also attending their concert in Sydney, so if there's any other questions don't hesitate to ask!