Can I get in trouble for doing this?

Yeah you are killing cats. You need to be sure your bait can only be eaten by rats or mice. What if a neighbor kid eats it? And yes, you could get in big trouble.

There are rat bait holders which are designed to prevent animals like cats from being able to get to the bait. They are universal.

If some DA lets their cat run around the neighborhood, they get what they deserve.

You need to look up your local laws. Where I live it would be illegal and you would be facing at least a fine if caught. It's not just cats you should be worried about poisoning, but also the wildlife.

While the risk of secondary poisoning is slim, it is also something you should take into consideration, especially if your poisoned bait is killing a lot of small animals. Even if it is allowed to put out poison like you're doing now, I would strongly suggest that you get some tamper resistant rat poison stations.

As far as I know it is legal. I am also in the US but most of these laws don't change much between countries. We have public utilities who put out poison rat traps at times and they are boxes that only creatures the size of rats can get inside of. That's a thought, too

Don’t know about Perth (I live in the US) but here you wouldn’t get in trouble as long as the poison was only on your property and it’s a legal rat poison product. If someone’s pet comes onto your property and licks it and dies it’s going to fall on the fault of the pet owner for not keeping their pet in their homes or yards. In the US, your property is yours to do whatever with (within legality).

pretty hard to poison a cat as the cat is meant to be locked up at night, if it wasn't on your property it wouldn't of happened probably not. Its only a cat they could all ways get another one if they liked it that much they wouldn't of let it roam around at night would they.