Often data was being used without customers realising, such as automatic software updates.
Roaming costs were an increasing concern for people as smartphones became more popular, said Vodafone general manager of business marketing Becky Lloyd.
"What happens is they've learned a whole load of new habits, like Facebook and maps and email.
"So people are doing it at home and then they go overseas and carry on that behaviour," she said.
Different apps used different amounts of data so it was often difficult to estimate just how much was being used, Ms Lloyd said.
Users were often scared abouthow much overseas data use would cost.
Vodafone had developed a system under which travellers could buy a data bundle and were notified when it had almost run out.
To avoid big overseas bills, Ms Lloyd also advised smartphone users to disable auto updates in Windows or anti-virus software, avoid websites with video or audio streaming, use the most efficient network, and block pop-up windows if browsing the internet.