A 6-year-old boy who was injured in a head-on crash by a cannabis-affected driver has been left with more than just physical scars, a court has heard.

The victim was admitted to hospital after the van in which he was travelling on April 14 was hit by Aaron Kahu Ashby (32), who later told police he lost control because he sneezed while lighting a cigarette.

The boy sustained a "severe gash" to his face, as well as multiple contusions to his arms, legs and neck, Judge Michael Turner told the Dunedin District Court yesterday.

He now suffered from nightmares, a fear of travelling in vehicles - particularly at night - and required "constant reassurance", the judge said.

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Ashby previously pleaded guilty to two charges of careless driving causing injury while under the influence of a drug.

His counsel Jo Turner said the man was becoming increasingly concerned about a family situation and decided to drive from his home in Balclutha to Dunedin.

Ashby attempted to enlist the help of flatmates but eventually decided to make the journey alone - contrary to the learner licence he held.

On State Highway 1 near Henley, the defendant's Honda crossed the centre line.

The driver of a Toyota - carrying three children - heading in the opposite direction tried to take evasive action but could not avoid the collision with the Honda the defendant was driving.

Both vehicles came to a rest beside railway lines, extensively damaged.

Ashby's car was so badly mangled that he had to be cut from the wreckage.

As well as the 6-year-old, an adult in the Toyota was also taken to hospital for treatment.

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He fractured a bone in the joint of his shoulder and required intensive physiotherapy in the weeks following the collision.

While Ashby was in hospital, a sample of his blood was taken and analysis tested positive for THC - the active constituent of cannabis.

Ashby admitted having used cannabis but said he had consumed it a few days before the crash.

If police had been able to prove his drug use had been behind the crash, more serious charges could have been laid, the judge said.

Ashby, who had no previous convictions, had also been hit hard by the incident, his counsel told the court.

He had an arrangement to pay off $7500 he now owed the victim's insurance company and Judge Turner also imposed reparation totalling $2577 (to be paid off at $60 a fortnight).

Ashby was sentenced to six months' community detention, 200 hours' community work and was banned from driving for 15 months.