Mr Cocker said the problems began when friends fell out or couples broke up - but still had access to each other's private pages and information.
"It happens quite a lot, and a significant proportion of bullying that occurs post-relationship break-ups would be avoided if people didn't share passwords or login details.
"Everyone has individualised log ons and connections and they should remain individualised.
"If you break up, then change your passwords."
A cautionary tale was the jailing late last year of 20-year-old Wellington painter Joshua Ashby for four months.
In what was described as an "irresponsible drunken rage", he posted a naked photograph of his ex-girlfriend on Facebook after the break- up of their five-month relationship.
The picture was initially visible to 218 of her friends but Ashby then made it publicly available and changed her password. It took 12 hours before the account could be shut down.
In the US, stories of fallout included:
* A spurned boyfriend in junior high school who tried to humiliate his ex-girlfriend by spreading her secrets by email;
* Tensions as boyfriends and girlfriends scour each other's private messages for clues of disloyalty or infidelity;
* Taking a former friend's cellphone, unlocking it with a password and sending threatening texts to someone else.
Mr Cocker said victims could lodge a complaint online with Netsafe service The orb or contact the server of the network.
"Over time you may be able to remove things, but unfortunately, once things are out on the net they can easily find a life of their own."
Relationship Services national practice manager Cary Hayward said social media was an easy way for a jilted partner to retaliate.
"One of the best things people can do if they know they are moving into a separation is to change their password."
Under a new proposal unveiled by the Law Commission last month, cyber-bullying victims could seek justice through an internet enforcer imposing fines, ordering apologies or even terminating the offender's internet account. Among other offences, the proposed laws would make it criminal to post "intimate images" online without the subject's consent and to "maliciously impersonate" someone on social media such as Facebook and Twitter.
Playing safe
* Advocates are warning not to share individual passwords on social networking sites.
* 30 per cent of teenagers shared passwords with their friends or boyfriend/girlfriend.
* Girls were twice as likely to share their passwords.
* Last year Joshua Ashby was jailed for posting a naked photograph of his ex-girlfriend on Facebook.
