I remeber that the first pig my brother adopted was pregnant. But it wasn't the pet store's fault; they were always up-front about the animals they sold and told us she was expecting. However that guinea pig died because of her age (about 9 mths old); apparently the breeder didn't tell the store this was her first litter and that she was bred too late.
They stopped buying from that breeder after they realized what happened.
That pet store actually took good care of their animals. They kept male and female animals separate and in big cages. They were even willing to separate animals that did not get along. We adopted 3 more guinea pigs from them, and all 3 were healthy, happy, and well-adjusted pigs. One lived for a number of years... despite our naive way of caring for him.
They also always had a local vet check their new animals and only used certain breeders. Sadly the store closed years ago.
Our area PetSmart is pretty good. I'm not sure if those are in Austrialia though. My only complaint is that they keep guinea pigs in small tanks no different from those they use for rats. On the plus side they always have their stock vet-checked and use more reliable breeders. I also have to give the store credit for housing cats (and rabbits) from our area's shelters and rescues instead of simply using breeders. That helps those animals find homes. Maybe they'll start housing guinea pigs from shelters sometime.
SuperPets is a bit of the oppostite of PetSmart. They keep their pigs (and ferrets and rabbits) in bigger runs, but theirs are often not as healthy. Plus they overcrowd their mice intended for snake food. I once counted over 50 in 2 cages that were connected. The poor mice completely covered the floor of the cages and were even in the space where the screen attached to the top of the cage. Thankfully the store has fixed that slightly; now they limit each cage to 15 to 20 mice.... still a bit crowded but much better than before.
(Sorry I can't say much about Australian pet stores; we just went thru something similar in having a pregnant guinea pig. I hope you didn't mind reading my post though.)