It sounds like Australia is run similar to Boy Scouts. In the US, it's girls only and it is rare to have a multi age/multi grade troop. I do know a few leaders who have done it and it is run similar to how you described.
For the majority, it is one grade per troop. Troops are formed based on geographical boundaries (usually within a school district) that are part of a larger Service Unit that is part of a larger council. We have girls from only one school because my tiny town only has one school per group. (One Elementary k-3, one intermediate 4-6, one junior high 7-9, and one high school 10-12.). Our SU actually encompasses all of the small towns (like ours) and a small part of the bigger city close by. I currently have the only 4th grade Junior troop in town. But, I know there are enough other girls interested to form a second troop for our grade level, the problem is NONE of those parents are willing to become a troop leader. That's part of the problem. They want the benefits of scouts for their child, but they are unwilling to do the work. Leading a troop is work. I work full-time outside the home, my daughter is in other activities and ye, I manage to lead a troop too.
A PP is correct, leader and co-leader MUST be unrelated adults. So, you must have two unrelated adults to run a troop...for the girls safety. And all adults involved must go thru a complete background check every two years.