The Department of Foreign Affairs and Trade can't pinpoint how much money it will save from removing access to consular assistance for Australian dual nationals and permanent residents in the countries in which they are citizens.
The measure was announced in Tuesday's federal budget under the banner of $50.5 million over four years in overall department efficiency savings.
Labor senators accused department officials during a Senate estimates hearing on Friday of having no evidence savings will be made by the restriction.
Department secretary Peter Varghese admitted the agency was unsure how many people seeking consular assistance are dual nationals.
"We don't have a comprehensive database on dual nationals," Mr Varghese said.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"On the basis of commonsense if you stop doing something you were previously doing you're going to save some money."
Department officials acknowledged it's not compulsory for people to report if they are dual nationals when seeking consular assistance, therefore data was incomplete.
"You make assumptions when you are doing costings all the time," Mr Varghese said.
Access for consular assistance will be restricted from 2017.
Mr Varghese said the list of exceptions is yet to be determined, and would be a matter for the next government after the election.
He expects natural disasters to be an exception and possibly old age.
