A 10-year-old boy managed to avert a tragedy yesterday after his nana passed out at the wheel of her car on State Highway 1.

Ryan and his nana Kathie were driving north between Meremere and Mercer just before 10.30am when she experienced a medical emergency.

He realised Kathie was slipping into a semi-conscious state, according to Counties Manukau Police.

"He soon swung into action and stepped over the centre console to step on the brake," police said in a post to social media.

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The car slowed down but Ryan couldn't remove his nana's foot from the accelerator. He waved to passing motorists for help as the car drifted toward the crash barrier - and the Waikato River.

"Thinking they were going to crash, Ryan managed to get the car to slow to a walking pace and jumped clear of the vehicle," police said.

A truck driver spotted Ryan waving, stopped his truck, ran to the crawling vehicle and brought it to a halt.

But once St John Ambulance arrived, the truck driver left the scene. Counties Manukau Police have shared the "heart-warming tale" from their Highway Patrol team in an effort to track down the hero driver who helped avert a disaster.

"Today, Ryan and Kathie came in to see Sergeant Fleming at the Pokeno station. We're pleased to report that Kathie is recovering well – Ryan's heroic effort helped to save his nana's life," police said.

"We'd still like the public's help - we want to get in touch with the truck driver to thank him for his actions.

"Are you that driver? Or do you know who they might be? We'd love to hear from you!"

POLICE SEEK HERO TRUCK DRIVER: We want to share this heart-warming tale from our Highway Patrol team – but we need your...

Posted by Counties Manukau Police on Tuesday, 8 October 2019