Before you do to Auslan, get her onto compics. It's quicker, easier and you & biomum don't have to learn another language in order to communicate with her. She's exactly the right age too.
The trouble with ANY of these things (speech, Auslan, compics) is the issue is about communication. I was talking about it with difficult child 3 yesterday, about receptive language, expressive language and speech.
Think about a pet dog who is well trained. The dog recognises basic commands such as "sit", "heel", "shake hands". You say these, the dog responds with the corresponding action. In other words, the dog has some level of receptive languavge. Of course the dog can't respond verbally (apart from barking) so expressive language from the dog is non-existent.
Now think about a parrot. For most birds, what they say is pure sound, it has no meaning for them. But it IS possible for some smarter parrots to learn words with context. For example you can teach a cockatiel to say the word "millet" when it wants millet. This would mean that for that word, it has receptive language (it understands when you say the word, that you mean 'millet') but it also uses the word appropriately, knowing what it means.
Then you have the sign language (and other forms of symbol communication). We know that apes for example can learn Ameslan (or similar) and communicate using it. Although the apes do not talk (because they simply don't have the complexity of vocal apparatus with which to do so) they can use sign language to carry on quite complex conversations, for animals. We also know from recent events in zoos that orangutans at least are capable of planning complex strategies in their heads, then carrying out plans step by step. But again, no speech.
The problem with autism is not just speech delay, but language delay. Unless you've lived with it, it's hard for people to understand the full ramifications of this. A child with language delay can really seem retarded (to use an outdated term) because they simply do not respond to instructions, they seem to not understand. difficult child 3 did not even understand or recognise his own name. It wasn't lack of exposure to speech that was the problem - he had the same exposure, the same level of stimulation there, as his siblnigs. It was as if he was deaf, except that he wasn't lip-reading either. He was simply unresponsive and also couldn't make himself understood. He would take us by the hand and drag us to the fridge to indicate he wanted something form in there, but it is not quite the same as communication. It was more like him using us as a tool, like dragging a chair up to the cupboard when he wanted something off a high shelf. He did that all the time!
Compics are like a step in between. They are an abstract representation (and hence communication) but they also are a picture of the item and therefore also easier to understand what they are. difficult child 3 might have grabbed my hand and dragged me to the fridge, but with a compic he could flip to the card showing the drink of orange juice and point to it. It would produce the same result for him, especially if we rehearsed it and taught him.
To teach him - he might grab my hand and drag me to the drige. I would then open the fridge and he would grab the orange juice off the shelf.
So I would give him a small amount of orange juice (because if I didn't after he had showed me what he wanted, there would be a tantrum and anything I tried would not get understood). Then I would get the compic cards, sit with him while I flipped to the one that showed orange juice, then I would say, "Orange juice" and pour him another half glass of it.
In so doing, he could associate it with his usual method of getting his message across, we could cross reference it and help link it to this new (and broader) method of communication.
Compics are worth trying. Have a look for them online, but try starting with what you can find on tis link.
http://www.mayer-johnson-symbols.com/websites.htm
Another signing method you could look into is Makaton. If you saw "Black Balloon", they used Makaton signing in that. I talked to the mother of the young man that character was based on, we talked a lot about what worked for her. You would use Makaton if compics didn't click.
For now, download some compics and make some cards for her. You print out the compics, stick them to cardboardd and maybe laminate them (or cover them with clear contact). Whatever you choose to do - what works for you. You then punch a hole through one corner and hang them from one of those plastic shower curtain rings or something similar. It's something you can do easily now.
WHile you're doing this, you get her name down for Speech Pathology. First an assessment, next some ongoing support and therapy. If you can't afford private, try accessing it through either Community Health or DOCS. Again a good excuse to get DOCS involved - they can open doors for fast access to these services. And that ofcourse is your only motive - to get her the help fast! Because OF COURSE there's nothing wrong with how this mother is/has been handling things...
Marg