'Miniature' Australian Shepherds do not exist in Australia and are not likely to in the near future.  The reasoning behind this is a bit long winded.

The United States Australian Shepherd Association or the Australian Shepherd Club of America are the breeds parent clubs in their country of origin.  It is the stance of both that they do not support the purposeful breeding of Australian Shepherds against the breeds standard, in this case for a non-standard size, as it is contrary to the breeds function and goes against the preservation and protection of the breed.  Neither club recognizes nor registers size varieties within the breed.  Instead Aussies like Golden Retreivers, German Shepherds, Border Collies or any number of other breeds only come in one size range with the preferred height for males is  51-58.5 cm (20-23 ins) and females  45.5-53.5 cm (18-21 ins).

The reason for that is the height of the Aussie is a function of their development as a stockdog. To work in the manner and environment for which they were created requires a dog of the size and strength to handle the job they were designed for.  A dog which is too small is both in danger of the stock, ineffective and left it's owners life in danger.  Likewise a dog which was too large lacked the agility, limberness, stable center of gravity and ability to heel low enough not to be kicked.  While there are smaller herding breeds (eg Shelties) they were bred to work in different conditions than Aussies who had to work middle of a several hundred head of range cattle in the rugged, scrubby conditions of the western US on a few thousand acres for weeks on end.

In the early years, those people who wished to breed a smaller than standard version started breeding separately from the recognized registries.  While they did use some registered Australian Shepherds in their foundation stock they also used non-registered dogs of unknown origins, crossbreds, as well as crossing in several other breeds to get smaller dogs.  The result was that these dogs offspring were no longer purebred, registered Australian Shepherds but a non-registered, non-purebred breed in the making.  There was a lot of division between people within that group who wished to petition the
ASCA and USASA to recognize a miniature size variety within the Australian Shepherd breed and register the resulting non-registered/nonpurebreds, while others including the 'founding breeder' for the small dogs wished to be recognized as a different breed that simply derived part of it's heritage from the Aussie.  The ASCA and USASA have always maintained the stance that there are no size varieties nor can they compromise the stud books to take in as purebred Australian Shepherds dogs who are several generations removed and whom have non-registered Aussies, mixed breeds and other breeds in their
background.

The 'miniature' is therefore not an Australian Shepherd.  There is some work in their country of origin to have them recognized under the name of Miniature American Shepherd but until they have full registry there with the recognized registering body (American Kennel Club) the Miniature American Shepherd is not able to be registered with our governing national body the ANKC.

There are Australian Shepherds which are on the smaller end of the standards heights in Australia as well as those who are smaller than standard and they make lovely pets.  They are however not miniature Australian Shepherds but small Australian Shepherds.  So be aware that a smaller but highly active individual will be less suited to a smaller space than a standard sized dog who is more placid natured.