Delta is the first time Australia has seen widespread infection in healthy people.
I'm not totally sure that's the case - it's at least something you can't just aver as self-evident. I'd guess that those most likely to be infected early on are going to be people who travel or those who come most into contact with them (service & tourism workers).
But I also think it's a moot point. In the context of this thread - which is about how to move forward once we've accepted that containment is off the table, and especially about the risk that poses to the healthiest members of society (kids) - it doesn't make sense to cite numbers that you yourself acknowledge overstate the risk of COVID by being biased towards the most unhealthy members of society.