There is a very good cat tranq available here, called Clomicalm, bound to be available where you are as Australia last to get everything.
Very effective, but like with ppl, you need to work out what is causing it, otherwise cat be on tranqs whole life, like us
Not really matter if cat on tranqs. PM any time, me cat breeder, former dog breeder, lots of experience. Unfortunately what you describe is quite common in Siamese. If you PM me i can put you in touch with lots of Siamese breeders who may be able to help. i/we breed abyssinian cats, member of Governing Council here, much vet help available.