Identity crime is on the rise as criminals become cyber savvy and fish around on social networking sites for personal information, experts say.

Already the cost of identity crime is put at up to $200 million a year in New Zealand. Facebook, Bebo and Twitter continue to gain popularity but having weak passwords and posting personal information make the sites easy prey for criminals.

Privacy Commissioner Marie Shroff said recent reports suggested a huge increase in social networking sites being used for identity fraud.

If someone got enough personal information, including your name, address, date of birth, bank account number or employment details, they could apply for a credit card or loan in your name, she said. "Your personal information has value – don't let someone else profit from it."

Internal Affairs said identity crime was increasingly used to facilitate other crime. identity crime – including creating a false identity with fictitious data and counterfeit documents, stealing the identity of another person or colluding with someone to get access to their data or documents – is believed to cost between $132m to $200m to the economy annually.

Detective Sergeant John van den Heuvel, of the National Cyber Crime Centre, said people freely posted information on the internet – from their date of birth, pets names, where they worked and what they did. Often security settings were not activated.

"It's a huge amount of personal information you wouldn't stop and give to the person on the street." Criminals could pose as a friend of a friend and become accepted into the inner circle, sending scam emails for money or gathering details.

"It's clear that criminals do `fish' these social networking sites to gain evidence," he said. "They can monitor your activity if you're sending out tweets when you're not home or excited about your overseas holiday. Who are you telling this too?"

Sites could be hacked because of weak passwords.

Green MP Sue Bradford's Twitter account was hacked last year with messages posted about her sex life; she has since changed her password.

Dutch website PleaseRobMe.com was started last month highlighting the danger of sharing too much information and how vulnerable people can be to burglars. It lists "recent empty homes" showing tweets from people broadcasting where they are.

This week is Fraud Awareness Week.