Screen shots posted by Lisa Maree Moynihan attacking people she claimed ripped off her son were shared more than 100 times.

A mother's "public service announcement" after her son got "ripped off" trying to buy cannabis has backfired.

Lisa Maree Moynihan, 35, was "frustrated" her son had paid $150 for cannabis, but never received the drugs.

Moynihan named her son's alleged scammers in a Facebook post, focusing on one woman in particular, saying she wanted "public shame for this muppet" until she paid her son back.

Moynihan posted screen shots of messages between the alleged scammers which showed they plotted to take money from her son.

"I can rip someone off bro, $150?" one message said. "That's gas bro, a sesh and a feed."

Moynihan, of Blenheim, said in her post in August the woman and her three friends were "setting up your teens to be robbed".

SUPPLIED Moynihan said those behind the messages were "setting up your teens to be robbed".

Her post also said where the woman worked.

It was shared more than 100 times, and comments showed "a majority were in support" of Moynihan, a police summary of facts said.

Police said the woman named in the post was too upset to go to work, fearing people would confront her.

She was afraid she would lose her job, police said.

When spoken to by police, Moynihan said she did not know she could face criminal charges for a Facebook post, or that the post would be shared so widely.

Moynihan told police she wanted to "raise awareness" and "do the public a service", and hold the victim accountable for her actions.

She felt the only way to do that was through Facebook, because she did not want to go to police out of fear her son would be judged and ridiculed for his bad decision, she told police.

She admitted her son used drugs, and disapproved of him purchasing cannabis, she said.

Police charged Moynihan with causing harm by posting a digital communication, under the Harmful Digital Communications Act 2015.

She admitted the charge and appeared for sentencing at the Blenheim District Court on Monday.

Her lawyer John Holdaway said Moynihan was upset and frustrated about her son being "ripped off".

She twice approached the woman and asked her to pay the money back, Holdaway said.

The first time the woman said she would repay Moynihan's son, but the second time she told Moynihan "to go away, in no uncertain terms", Holdaway said.

That encounter prompted Moynihan to publish the post, he said.

Moynihan was a vineyard worker and could pay a fine over time, but already owed nearly $5000 so community work might be more suitable, Holdaway said.

She had previous convictions for wilful damage and assault.

Judge David Ruth said the new legislation was supposed to stop people from hurting people on social media, which was "impersonal and cowardly".

He recalled sentencing one of the first people charged under the new law to 15 months' imprisonment, which the defendant unsuccessfully appealed, he said.

"It's a very serious crime. It's one you could go to jail for, and you need to understand that."

Judge Ruth convicted Moynihan and sentenced her to 80 hours' community work.