Is there an Android Equivalent to iMessage?
Since you are in Australia, I would suggest trying t0mm13b's suggestion of Viber. You will have to "Validate" your account with Viber in a location that you have mobile data connection, according to this article, first, but then you should be able to use the service just fine on wifi.

If available, Google Voice is a good option. You will be able to send and receive SMS from it on both mobile network and wifi.

It is probably your best option as it doesn't require the other person to have a different application to be able to receive the messages. Most of them will require the person receiving the message to have the same application.

Google Voice doesn't require the other person to have a special application. It will send the messages as SMS. It will, however, require you to use the Google Voice application to send the messages.