A Nelson woman has been found guilty of offensive behaviour after rat poison was rubbed on National MP Nick Smith.

The politician had claimed that Rose Renton and another protester rubbed rat poison on him while he was at a Saturday market in Nelson in September.

She was charged with offensive behaviour and Justices of the Peace have found her guilty of the charge, RNZ reported.

Renton came to national prominence in 2015 when her son Alex became incapacitated in hospital with a prolonged seizure.

Then Associate Health Minister Peter Dunne approved a petition to the Government from Alex's family to allow him to be treated with a medicinal cannabidiol oil.

Smith said he had poison rubbed in his hair and clothes at the Nelson market incident. However, Renton said she only touched him lightly.

Smith said today that he welcomed the guilty verdict.

"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," he said.

"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."

Smith said he was worried about the number of ugly incidents which had occurred as the use of poison for pest control increased in New Zealand.

"I have previously faced death threats as Minister and there have been several dangerous incidents and threats against Department of Conservation staff."

He encouraged Conservation Minister Eugenie Sage to increase education about the key role poisons played in protecting native wildlife.

- Nelson Weekly