One of New Zealand's largest unions says the Employers and Manufacturers Association (EMA) is "scaremongering" when it claims employees should face legal action for complaining about their jobs on Facebook.

The Engineering and Manufacturers Union (EPMU) has come out strongly against the call from the EMA, saying that prosecuting people for what they say online comes "dangerously close" to impinging on fundamental rights, such as freedom of expression.

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

EPMU lawyer Greg Lloyd said the EMA was scaring employers into thinking they should prosecute people for comments made on social networking sites.

"You can complain about your workplace in any environment and just because it's in an electronic form shouldn't make any difference," he said.

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.

- NZPA