The coach of a national sporting code is facing charges after an unhappy mum turned amateur detective.

BMX high performance national coach Ken Cools has been charged with careless driving causing injury after an accident involving an aspiring navy marine engineer.

Canadian Cools and Don Simich, 22, collided head-on in Pukekohe on November 11 last year. The accident left Simich with a shattered femur and a long path to recovery before he is able to return to sea.

Now, police have charged Cools after he allegedly crossed the centre line. He is remanded to appear in Pukekohe District Court on Wednesday, after pleading not guilty to the charge.

Cools, who was recruited to the BikeNZ team in March 2008 as national BMX coach, refused to speak last week but, in the November issue of BMX Racing News, he wrote about the Runciman Rd crash.

He said it was "one of scariest and worst days of my life" and described Simich as an "out-of-control driver who lost control of his car while rounding a curve".

He wrote he was taking his daughter and her friend to Chipmunks Playland when the accident happened.

"Out of nowhere some guy came flying around the corner going way too fast in his WRX STI," he said. "Since he was going so fast, he couldn't hold the speed just in his lane so his car had swung over into my lane.

"I saw him at the last minute and reacted by deviating into the open side of the road, which was his lane. Unfortunately, he tried to correct himself at the same time and we met head-on. He was travelling over 100km/h so it was a nasty collision."

Cools wrote of his relief his daughter and her friend were not seriously injured. Cools said he broke ribs, fractured a wrist and felt like he had been in a fight with Mike Tyson.

He said the experience was particularly fraught as his wife, Jenn, was expecting another child that month. "Thank God she was not with us," he said.

But Simich's mother, Raynor, said charges came after she questioned police over damage to the cars because she believed there was a presumption her son was a boy racer because of the type of car he drove.

She said there were three police officers on the scene and none took statements at the time. "The police blamed Don straight away."

She said she had visited Cools in the aftermath of the accident to see if he was okay. She said Cools was distraught and apologised for his role in the accident.

She then received a letter saying no one was being charged and, when she next visited Cools, was told he had retained a lawyer and would not speak to her.

Raynor Simich said the letter prompted her to contact the wreckers' yard to gain access to her son's car to see the damage.

She said she was told Cools' car was also there - and was being taken away to be crushed the next morning.

She rushed down and examined the damage to both cars and believed it told a different story about the accident, so rang police saying: "I'm panicking about this".

The police halted the car crushing, asked the Serious Crash Unit to visit and followed up with charges.

Simich said he was "broken" by the crash. "I saw him as he was just starting to come across the centre line. He saw me at the last second and he swerved into me.

"He smashed into me and we both ended up on my side of the road.

"I was trapped in the car and had to be cut out. I shattered my femur in five places and had to have major surgery to have a titanium rod inserted from hip to knee. I have spent the past six months on crutches."

He said his injuries had hindered his career as he was unable to sit the exams he had been scheduled to complete.