A North Canterbury teenager surprised police twice at the weekend - first when she phoned them to report cannabis growing in her back garden and second when she told them she believed either one or both of her parents were responsible.

A police spokesman said the 18-year-old called police on Saturday after finding two large cannabis plants "nestled amongst the shrubs" in the Woodend property.

The girl's parents were away for the Easter break and she had found the plants with visiting friends.

The spokesman said she made the decision to phone police immediately.

Police seized the plants that day.

He said the plants were about 1.5 metres tall and were "near harvest".

The spokesman applauded the girl "for her stand against drugs" in the "catch-22" situation.

"She was confronted with an ethical dilemma. So often we say the older generation are wiser than the younger. Maybe we didn't get that right."

He said the parents would be interviewed by police in coming days, and they were aware the plants had been seized.