The new two-lane bridge at Taipā was designed with a platform for leaping into the river, but the NZTA has now erected signs warning that it would be dangerous to use it.
Signs "strongly advising" parents to discourage children from jumping from the bridge are reportedly to be erected.
"While the original plan was to provide a jumping/swimming platform, a recent health and safety review by the project team found that it would not be possible to provide such a platform where it would be safe to jump at all times," NZTA portfolio manager Stephen Collett said.
"The NZ Transport Agency does not condone jumping off the bridge, as it is potentially dangerous, with the possibility of injuries from submerged hazards, rocks, floating debris and passing boats.
"The risk of injury from submerged hazards and rocks is particularly significant at low tide."
Construction of the new bridge began in 2017, the design including a platform "to acknowledge the bridge's place in the community as a summertime resting and diving spot for kids of all ages".
Opposition leader Simon Bridges said he was proud that the project, initiated by National, had been completed, but it was "a real pity" that there would not be a jumping platform.
"That was the fun part of the bridge project that really captured my imagination," he said.
"I realise it's all about transport, but kids will be kids, and will want to jump off it, and they're going to anyway. I might pack my board shorts next time I'm up that way and give it a go, as long as I'm not going to be arrested."
Whatuwhiwhi resident Vanessa Raui said all four of her children had jumped from the bridge over the years. Now aged between 22 and 14, Ezekiel, Shalynd, Fleurnik and Te Atenga began jumping as students at Taipā Area School, and had never been hurt.
"It's always been a summer pastime for the kids, you see them on the weekends with their families. That's what creates memories for them. They always talk about jumping off the bridge and swimming with dolphins. There's not many kids that can say that," she said.
Taipā Area School principal Doreen Bailey said the school had held annual "controlled jumps" over the years, where safety measures were in place, but would now have to reassess that.
"As a school we have to keep health and safety in mind, (but) if you build a bridge in the North people are going to jump off it. "