Added - she is actually young yet, and may grow a bit more. At nine months, they should be about 4/5 their full size, and there is still time for her to grow a bit more. You should have been given registration papers, in which you should be able to research who her parents are, and whether they are registered Aussies.
Here is the breed standard according to ASCA, the parent club:
http://www.asca.org/aboutaussies/breedstandard
She is well within the standard, and some can be very petite even if they are full bred Aussies and not those horrible little cross bred "mini's". The dog in my avatar is from some of the oldest bedrock lines, and she's only 36 pounds. If she has a high metabolism, it may also be hard to keep weight on her - my girl looked like a Greyhound in the winter, although she was eating over three cups of puppy food - still getting lunch- up to the age of a year. Some can just be sort of lanky.
The show lines can be much stouter and hairier, but in the beginning, the stockdogs were very light on their feet and not meant to be enormous dogs with heavy bone. That is very possible with your dog. I'd be curious to know where you got her.
Her skeleton will be growing until she is almost a year, and then she will fill out more by the time she is two- but she would be expected to reach her full height by one year old or so. At her age now, her rear will be higher than her front - they grow unevenly.
DId you see her parents?