Concern over teenagers doing bombs into filthy floodwater at Porirua Railway Station saw the police called to cordon the area.

Mass rainfall on Thursday created a mucky pool from the platform down into the subway tunnel, with the ramp railing being used as a diving area for the night-time jumpers.

After seeing a video on Facebook of the bombers, Porirua mayor Nick Leggett said he initially thought it was just a bit of fun, but ended up calling the police.

Leggett posted on Facebook to say he had shared the video himself, thinking it was "harmless", but after seeing copy-cat videos asked police to patrol the area to put a stop to it.

"Clearly it had given some other kids some ideas."

He was concerned about the contamination of the floodwater. "If there is sewage in that water then there would be a health risk," he said.

Civil Defence warned people on Thursday to stay out of floodwaters because of potential sewage contamination.

Leggett said he was also worried about electrical currents in the water because of subway lights and wiring, as well as the risk of kids getting sucked under the subway.

"I saw the video, and a hundred thousand other people saw it, and you sort of think 'oh yep that's a bit of a laugh,'" he said.

But late at night when there was no one around, the risk in jumping in was not worth it, he said.

Police were called in to cordon-off the area and no jumpers were hurt, he said.

In the future, Leggett said the area would be better monitored.

"Next time if that was an issue we'd know very quickly and we'd cordon that off far more quickly."

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