A retired judge with considerable experience in laws surrounding online issues says those who share posts which break the law could be legally liable themselves.

The comments by retired Judge Dr David Harvey, who is Director of the New Zealand Centre for ICT Law, come after Deputy Prime Minister Paula Bennett threatened legal action over an online post accusing her, without evidence, of several historical criminal acts.

According to a cease-and-desist letter sent to a North Island man by lawyers acting for Ms Bennett, a post and video were published on June 30 containing "material highly defamatory of the Deputy Prime Minister".

The post has been widely circulated and the letter states it has already had more than 5400 shares.

Dr Harvey told 1 NEWS that people who hit share on such posts should be aware they could find themselves at the sharp end of a lawsuit.

"They themselves could be deemed to be publishers of the information and be liable for whatever legal wrong they have done," he said.

"It could be a breach of confidence, it could be intentional infliction of emotional distress ... it could be the sharing of information under the Harmful Digital Communications Act or it could be defamatory."

The letter from Ms Bennett's counsel said "not only do these allegations very seriously defame the Deputy Prime Minister, but she, and we as her lawyers, will regard it as a form of harassment".

"You should immediately remove this content."

The lawyers acting for Ms Bennett say that further action could be requested in future in terms of remedial action and also say a restraining order could potentially be requested.

The letter emphasises Ms Bennett's "categorical rejection" of all claims made in the post.

No corroborating evidence supporting the claims, which cannot be repeated by 1 NEWS, has been produced by the man.

Ms Bennett's lawyers copied media organisations on the letter to the man warning them off publishing the accusations.