Why did one pullet start laying, and the other one stopped?

Hello and

How old is your Australorp? For all we know you may have been sold a pullet or hen who's older than you were told (happens all the time, my "point of lay" Isabrowns were actually over year old, lol)... It's normal to stop laying when coming to a new place but not all hens do that, and chances are she would have stopped sooner if that was the case.

Another possibility is that your diet is richer than what she was on, so her body has called a cease-lay in order to rebuild itself, making better cells to replace its older, inferior ones using this new, superior nutrient source. If that's the case she will do some mending for a while and then kick back into lay, and the eggs will be higher quality than before.

Might also be moulting time for her, even if you're not seeing the stereotypical bare rump. In my experience only birds on a bad diet, or birds with a strong genetic predisposition to that, will develop bare patches when moulting, whereas all the rest will lose random feathers and replace them in a mixed and staggered pattern, so at no time are they showing exposed skin. This I consider a more efficient trait as they consume less food, have no heating issues, and quite often keep laying through the moult, or don't stop laying for anywhere near as long as a hen who moults bare.

Another possibility is that she's contracted something, even though she doesn't look sick. Possibly from the other hen, even if that hen looks like she's in the prime of health, she could be a carrier and not susceptible to what she's carrying, or perhaps resistant to it.

And if they're free ranging she may be laying in some hidden place... Just some ideas.

Best wishes.