More information on the importance of the mobile phone as a banking interface emerged this week. Yesterday, NAB revealed that its customers funnel A$1 billion worth of transactions each month through their mobiles. Earlier in the week, Westpac disclosed that customers using their smart phones now account for 30 per cent of online activity.Speaking at the Cebit computing conference in Sydney, Ben Forsyth, NAB's head of mobile and emerging technology, said that of the bank's six million customers, two million had signed up for internet banking services. Of this latter group, 750,000 accessed these services from their mobile phones each month.Statistics shared by Google's industry leader for financial services, Mel Silva, meanwhile showed that 28 per cent of all mobile phone users had already made a purchase from their phone.However, Forsyth warned that, while the mobile phone was a popular device for shopping and spending, it introduced security risks for both banks and unwary customers. He said that as mobile phone transactions had become more valuable they had become the target of cybercriminals, who were targeting the phone with carefully designed malware (malicious software) intended to either steal customers' details, or to hijack SMS messages sent to phones for two-factor authentication of internet banking transactions. So far, Australians had seen just the "tip of the iceberg" with regard to the mobile malware problem, Forsyth warned."In app development you need to put security right up top and central," he said, adding that penetration testing - where teams of programmers on the bank's payroll deliberately try to break the security of a system in order to prove its robustness - was almost essential for organisations developing mobile applications. "The cost [of testing] is dwarfed by the potential cost of reputational damage [in the event of a security breach]", he said.In addition to conducting sophisticated design and testing of applications, Forsyth said it was also important to continually educate customers about the risks involved when using mobile devices. "There is this insane willingness to download apps without any due diligence about what the system is doing," he said.Forsyth also cited statistics showing that phones using the Android operating system were particularly at risk from being targeted by scammers and malware.