The mayor of Porirua, Nick Leggett, has asked police to cordon off a flooded part of the town's train station after a video of young people diving into the dirty water was posted on Facebook.
Heavy rain in the area caused water to pool up around the entrance to the subway tunnel.
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A video widely shared on social media showed two teenagers jumping off a station railing into the murky pool.
Mr Leggett said he was concerned the video could encourage imitators and asked police to block off the area and patrol the station.