A man has been refused bail over a crash that resulted in the death of a young boy in New South Wales' Upper Hunter region during October.

The 47-year-old driver of a four-wheel-drive was charged with manslaughter after a young boy died at the scene of the incident, while a teenage passenger was flown to Newcastle's John Hunter Hospital.

The man appeared via audio-visual link in Muswellbrook Local Court today, where it was alleged he was driving at excess speed with cannabis in his system when the crash occurred.

The court also heard doctors had told the man not to drive because he was suffering seizures — a fact he allegedly lied about to authorities.

In addition to manslaughter, the man was charged with several other offences, including dangerous driving causing grievous bodily harm and driving with an illicit drug present in his bloodstream.

The police prosecution opposed bail on four grounds — questioning his ability to attend future court appearances, a possible ongoing risk to the community, a potential for interference with witnesses, and the seriousness of the offence.

The accused's solicitor, Joshua Whale, argued these factors could be mitigated by imposing certain bail conditions, stressing his client had been compliant with the investigation and was in a vulnerable mental and physical state.

In refusing bail, Magistrate Kevin Hockey accepted there was an "unacceptable risk" and said it was an "extremely serious offence".

The matter returns to court in April.