A mother spoke last night of her shock at the fad of planking after her teenage son was photographed lying over train tracks.

A photograph of Tauranga Boys College student Reid Moodie, 15, was posted on Facebook after he posed for the stunt last Friday.

Last night, when contacted by the Herald, his mother Vanessa said: "I'm just in shock, I had no idea ... It's extremely dangerous of him.

"I absolutely would never have imagined that Reid would be planking," she said.

Mrs Moodie said her son had assured her that there was no train coming, but she said she would impress upon him how dangerous the stunt was.

"There'll be a big chat with him about it. But I think his father will have more to say about it to him than I would."

Tauranga Boys principal Robert Mangan said his school had tried to play down the planking craze so as not to escalate the competition.

However, he said he would look into the issue further at school today and would have a discussion with Reid and his parents about the dangers of his actions.

"I am concerned about the behaviour and the danger the young man may have placed himself in."

The photograph was the latest in a series of planking stunts - and not the first to involve children lying on a railway line.

A school pupil was videoed stretched across Middlemore Station in South Auckland while a train was approaching.

Secondary Principals Association president Patrick Walsh said the principal of a school near the station had told him about the incident.

"It was captured on a cellphone by another student and reported to the principal, who brought the student in and spoke to the parents."

The student had been disciplined.

Mr Walsh said at least four principals had contacted him about dangerous planking by pupils, including on the crossbar of rugby goalposts.

He had advised schools of the dangers of the fad and although he was not aware of anyone suffering injuries yet, he said: "I'm really concerned it's going to happen.

"My understanding of it is that the more dangerous the situation, the more kudos they get from their friends."

Rail safety campaigner and former cricket star Chris Cairns, whose sister Louise died in 1993 when a truck drove into a train she was travelling on, has added his warning.

"I am just speechless to hear about such utter stupidity. These kids need to get real and understand that the rail corridor is no place to be."

Cairns said the fine for trespassing on train tracks is up to $10,000.