Some hatcheries, Cackle for example, will sometimes add an additional chick that you ordered, male or female, especially if they hatched more than they need. They have a live chick guarantee and it doesn’t cost them anything extra to add an additional chick. It can keep customers happy if one dies. It can make customers happy if none die. Plus they kill any chicks they cannot ship. This is one less they have to kill.
This is different than adding additional birds just for body warmth during shipping. Practically all those are male.
They have experts that vent sex Australorps. They give you a 90% guarantee that they got the sex right. Their experts are actually better than 90% but they like to keep the odds in their favor. Still, you will occasionally get a male with your order. It may or may not be the extra chick.
A lot of people put a lot of faith in how tail feathers grow in at that age. I don’t. There are a lot of things that can act as clues to sex but most of them are just clues. Some cockerels develop pretty fast, sometimes I can spot a cockerel as soon as it hatches by leg size, posture, and attitude but a lot of cockerels develop more in line with pullets for a long time. Often, about five weeks different things develop so you can pretty much tell which are cockerels and which are pullets, but there are some chickens I’m still not sure of by more than twice that age. Australorp usually aren’t that hard. This thread talks about some of those signs.
How to sex chicks
https://www.backyardchickens.com/forum/viewtopic.php?id=48329
One pretty strong clue can be comb and especially wattle development. It’s not just size though size is a big clue. Color is an even better clue. If you have one developing red wattles compared to the others being pink not having any wattles, you have a male. At three weeks heavier longer legs and a more upright posture can also be pretty good clues. You get better at this with experience.
