There's frontline etc tick spot which works for about a week - go see your vet and chat with them. There's no substitute for keeping the cats away from the bush (and seriously, I strongly believe in keeping cats away from native wildlife, so if possible keep them inside overnight at least) and they will adjust to being inside cats after a while. Even with frontline, you'll have to check them daily with a tick comb etc. You could also give them tick rinse, but cat skin is very sensitive, so I would definately chat with the vet.