We had the same problem with our cats regarding the crystals in urine. The change/problem came about when we switched to Coles brand cat food. One of the cats is primarily an indoor cat as it is a hunter, and we noticed it was urinating more frequently, something we couldn't have observed if it was an outdoor cat. After some costly vet bills re crystals in urine and a switch to Hills, and then finally back to Whiskas, the cats are fine again.
Please don't buy Coles brand dry cat food.
Cheers, Tangrams Cat tolerator.
PS: Please also don't buy cats as they are so destructive to our wildlife and other fauna, killing millions of marsupials, birds and other wildlife each year.