ACMA's International Mobile Roaming Standard has taken effect since Friday 27 September, although the implementation of the standard's various components is staged.
All Australians travelling overseas will receive an SMS alert advising them when they start roaming and a warning that significantly higher charges may apply.
In addition, customers of Optus, Telstra and Vodafone will receive SMS messages informing them of the costs of using roaming services in each country they visit.
Medium and smaller telcos are building systems to allow them to send these SMS messages. In the meantime, customers of these telcos will receive information about roaming prices and an ACMA fact sheet on alternatives, when they first ask for access to roaming.
"The increasing use of smart mobile devices has meant that people using mobile devices overseas without careful planning are at risk of bill shock, possibly running to tens of thousands of dollars," Chris Chapman, ACMA's chairman, said.
"These warnings are important because international roaming services are usually priced differently from services at home.
"The overseas charges for receiving calls and casual use of data — for activities like maps, social media, email, browsing and uploading photos — are often expensive. And these further charges vary between mobile plans and between countries.'
"Optus, Telstra and Vodafone have all recently announced welcome changes to roaming charges, so talk to your provider and about the options before you travel and tailor your approach to suit your particular circumstances."