Hi Zigzag and welcome!
I will also underline that you need to seek a breeder who is doing MRIs for SM and using the SM breeding protocol. Your vetmay simply be unfamiliar with spotting SM -- even in the UK where the breed is very common, one research paper noted on average it took close to three years for dogs to be correctly diagnosed as vets will keep believing they are seeing allergies, skin problems, disk disease (also common in the breed) etc because this is a specialist disease generally requiring an MRI for diagnosis. Breeders who MRI on average have found about 50% of their dogs, with NO symptoms at the time, scan with a syrinx (SM). Once vets are familiar with it, they tend to find they regularly see it. My own vets in Ireland would also, at one time have said they rarely saw it. Once they learned more about it and also looked after my own dogs with SM, they now regularly refer cavaliers for positive diagnosis at the nearby vet school.
We have Australian members who have SM dogs (some, more than one) and there have been research projects involving a neurologist in Sydney. As the breeding dogs in Australia are all relatively recent imports from the UK and other regions with SM and are all mostly closely interrelated (the gene pool for cavaliers is relatively small) it would be impossible for there not to be the same incidence of SM in Australia as elsewhere (about 70% according to one 550+ cavalier study of asymptomatic dogs, by the time they are over 6).
SM is also known in Brussels Griffons and it is Australian griffon breeder Lee Pieterse that has pushed for MRIs and research internationally in that breed (which also has a high incidence of SM) and also helped raise awareness of SM in cavalier breeders in Australia too over the years.
It would be worth showing your vet this webpage from one of the Australian cavalier clubs, which makes very clear that there is CM/SM in Australia:
http://www.cavaliersnsw.com/index.asp?ID=11
Note there IS a programme set up for screening breeder's dogs. Hence I would not even consider taking a puppy in Australia from anyone not screening for SM as well as MVD.
Unfortunately almost every cavalier owner will deal with one or both of these conditions at some point, at some degree of affectedness. Carefully choosing a testing, protocol-following breeder can greatly reduce the chances of seeing a serious, early onset level of either MVD or SM. But any cavalier owner needs to be familiar with these conditions, be prepared to see them eventually show up in their dogs, and -- insure your puppy immediately with an insurer who covers genetic illness.
We all love this breed deeply but there are some sad realities about the overall state of breed health and right now, postponing onset or aiming for very mild conditions rather than avoiding them entirely is more the norm for both MVD and SM as they are so widespread in the breed.
Therefore choosing a health testing breeder for BOTH conditions, is critical.