Protestor Rose Renton​ does not regret rubbing rat poison on Nelson MP Nick Smith despite the actions earning her a criminal conviction.

Renton was found guilty of offensive behaviour following the incident involving the former Environment Minister near the Nelson Market on September 2.

She was fined $400 and ordered to pay court costs of $130.

MARION VAN DIJK/STUFF Nelson MP Dr Nick Smith says he's not surprised by the Greens' decision to oppose the local bill but welcomes the support of Labour.

Justices of the peace David Whyte and Betsy Wynne announced the result in the Nelson District Court on Thursday. It took them two hours to come to the decision as part of a defended hearing.

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Renton jointly charged with her former husband Guy Northover, who previously admitted the charge and has been sentenced.

In the decision, Whyte said Renton's choice to use real poison and record the protest showed her actions were premeditated and deliberate.

"The defendant rubbed a block of the poison onto Dr Smith, the defendant threw the poison into the caravan and onto the table outside the caravan, she also vigorously rubbed the poison onto a chair next to the table and the carpet inside the entrance to the caravan."

Whyte said members of the public that were near Smith's caravan at the time were exposed to the risk of the poison.

He said the behaviour was found to be offensive when Renton spread real poison on Smith and disrupted public order.

The incident occurred on the day of an aerial poison control operation in the Brook Waimarama Sanctuary.

Outside the court, Renton said she stood by her actions.

She said the decision raised questions about the use of poison, if her actions in a Nelson carpark were deemed to threaten the public's safety but Brook Valley residents' concerns about the safety of the aerial operation had been ignored.

"Do I regret it? No I don't, simply because it's a conversation that is long overdue and should have been had a long time ago."

Defence lawyer Sue Grey said while the decision was disappointing, it was also very detailed and highlighted several issues, including how the community had input into the way poisons were used.

In a statement, Smith said he welcomed the guilty verdict and he was "delighted" the matter had been resolved.

​"People are entitled to express their opinions, however, throwing and rubbing poison on another person has been rightly confirmed by the court as offensive behaviour," it read.

"The Nelson Market is a friendly weekly event for locals and visitors and we need to protect its reputation as a peaceful place where diverse opinions are respected."