In Canberra, the following veterinary practice is happy to be recommended as knowing something about cavies: Inner South Veterinary Centre, 47 Jerrabomberra Avenue, Narrabundah phone 6295 0770. 

In Sydney, there are the following:-

 Allambie Veterinary Clinic, 92 Allambie Road, Allambie Heights, phone 9905 0505.

Baulkham Hills Veterinary Hospital, 332 Windsor Road, Baulkham Hills phone 9639 6399

Birds & Exotics Clinic, Shop 3, Block D, 1 Hunt3er Street, Green Square, phone 9319 6111

Elanora Veterinary Clinic, Shop 2, Kalang Road, Elanora Heights phone 9970 6647

Springwood Veterinary Clinic, 12 Ferguson Road, Springwood, phone  4751 8888

University of Sydney Avian, Reptile & Exotic Pet Hospital,   415 Werombi Road, Camden phone 4655 0798

If anyone else knows of other vets who have expertise in cavies and are willing to be recommended, please send me the details.