Free SMS Providers
Unfortunately, there isn't any completely free SMS providers in Australia. Some other countries have much cheaper SMS rates (for example, the USA) so services can offer SMS messages for free subsidised by advertising.
Here in Australia, even the massive providers have to spend quite a lot to send an SM, hence why there's no providers offering it for free.
Don't lose hope though, as there are providers who offer free trial and small packages that are very affordable.
Free Trial SMS Providers
Most Australian SMS providers will offer some kind of free trial, though you can often only send to your own number.
MessageMedia provides 25 free SMS messages in a 14 day trial, and it great for a quick one of use case.
Other providers such as BurstSMS also offer free trials.
Free SMS API Gateway
Looking for a free message service API? Most providers that offer trials will also allow access to their API, though some providers will gate this behind making a purchase.
You'll need to make a purchase if you want to continue sending, because of the cost of messaging in Australia only the first few messages are free.
Receive SMS Free in Australia
To receive SMS message, you'll need a virtual or dedicated number. No Australian providers offer free dedicated numbers, so you'll have a purchase one. Prices generally range from between $20.00 AUD and $50.00 AUD per month.
No free trials are offered due to abuse of these numbers, so you'll need to fork out if you want to receive messages.
Once you have the virtual number, receiving messages is free on most providers, though some do charge between 2 and 10 cents per incoming message.