UPDATE 12/03/2017 11:20AM: The issue with iMessage experienced by some customers is now confirmed resolved. If you switched iMessage off you can now switch it back on and it will be working as normal.
We're currently investigating an issue where some customers in South Australia and the Northern Territory are unable to send and receive messages using the Apple iMessage service. Calls, Data and SMS appear to be unaffected.
In the mean time, by switching iMessage off on your iPhone, you will be able to send and receive standard SMS messages as normal (standard SMS costs may apply, depending on your plan).
You can do this by going to:
Settings > Messages > switch iMessage to off
We apologise for any inconvenience this may cause and we'll continue to update this article with further info as it comes to hand.