Employees venting workplace anger to their Facebook friends should face legal action, the Employers and Manufacturers Association (EMA) says.

EMA employment services manager David Lowe said the use of social media was untested in employment law but employers should take action if employees badmouth them online.

"Some employees continue to say things on their social networking pages forgetting it isn't private. Businesses must not sit back and allow their reputations to be sullied by the thoughtless comments of employees or ex-employees."

Mr Lowe said the EMA received calls nearly every day from businesses which had found a derogatory remark about the manager, customers, work colleagues or the company itself on a Facebook page.

"Many people don't seem to make the connection between age old legal principles and the modern world of Facebook."

Mr Lowe said business needed to keep an eye on the sites to see what was being said about them as both positive and negative comments could be influential.

Even former employees or those overseas could face potential legal repercussions.

A Council of Trade Unions spokesperson said employers needed to differentiate between their employees grumbling to their friends and running a defamatory website.

"Employers need to make any policy on this clear to employees just as they would in any other matter. The way they deal with this needs to be in accordance with contracts."

The spokesperson said any action needed to be in proportion.