An alleged drunken driver was unable to leave his hospital bed for his court appearance to face charges for killing a young mum and her toddler in a head-on smash over the weekend.

The 19-year-old accused was due to appear today in the North Shore District Court on two charges of driving with excess breath alcohol causing death, and one charge of failing to stop at the scene of a fatal crash.

A Northland mother and her toddler were killed, while another child was seriously injured, when their vehicle and another car collided head-on along State Highway 1 near Oruawharo Rd in Topuni at about 9pm on Saturday.

However, a duty lawyer told the court today he had spoken to the accused in Auckland Hospital and said he was not in a fit enough state to give instructions to the lawyer.

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The accused was also unlikely to be released from hospital this week, the court heard.

In the teen's absence, Judge Nevin Dawson remanded the teen in custody at the hospital until next Monday, when the case will be called again in the Whangarei District Court.

The 19-year-old was also granted interim name suppression by the judge.

Court documents viewed by the Herald show the teen allegedly blew a breath alcohol reading of 768mcg of alcohol per litre of breath.

From December 2014 the alcohol limit for drivers aged 20 years and over lowered from 400mcg of alcohol per litre of breath to 250mcg. The blood alcohol limit also lowered from 80mg of alcohol per 100ml of blood to 50mg.

For drivers under 20, however, the limit is zero.

Early police investigations revealed the 20-year-old mum from Kaiwaka, her two children - both aged under 3 - and a man were travelling north.

The injured toddler was flown to Starship children's hospital in a serious condition.

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The seriously injured man in the northbound car was also airlifted to Auckland Hospital by the Auckland Westpac Rescue Helicopter.

Two people were in the other vehicle heading south when the crash occurred.

The driver of the southbound silver sedan allegedly failed to stop and was later arrested after being located alongside the second person who was in the car near Auckland.

Diversions were put in place through Mangawhai, and SH1 in Topuni was closed until 3am yesterday.

The police serious crash unit examined the scene and investigators are appealing for witnesses to come forward.

Detective Sergeant David Hamilton said people driving on SH1 on Saturday evening, particularly in the Maungaturoto and Kaiwaka areas, who saw the silver sedan before the crash are asked to contact police.