Road workers were shocked when they saw children playing on a motorway overpass in South Auckland last Thursday.

A concerned contract worker for the NZ Transport Agency took the photo of five young children playing on and hanging off the Hall Ave footbridge over the Southwestern Motorway at Mangere.

He sent the picture to his traffic manager who then forwarded the photo to Manukau Harbour Crossing Alliance stakeholder co-ordinator Queenie Manga.

Ms Manga was very concerned about the children's behaviour and has tried to find their parents.

"They're sitting up there, and they've got that sign behind them but the sign only goes up so far and then one of them was standing up on the railing.

"So we wanted the stakeholders [their parents] to be aware of this behaviour and the footbridge isn't a place to loiter. I mean, it's not a playground. Something could seriously have happened to that child standing on the barrier."

Ms Manga said she did a letter-drop warning local residents of the children's dangerous behaviour and notified police.

NZTA's State Highways manager for Auckland, Tommy Parker, said this was the first time the NZTA or its contractors had seen this behaviour on that overpass, but he was concerned.

"Motorways are always fast, and always dangerous.

"The consequences could be heart breaking if a child balancing on a guard rail dangerously is distracted by a fast-moving vehicle or a sudden noise from an exhaust or a horn."

He said the children were scared off the overpass when another contractor warned them of the danger.

Mr Parker advises parents to keep a close eye on their children, especially now with longer evenings giving them more time outside.