Police in New Zealand have stopped a car driven by a seven-year-old boy while his grandfather was more than twice the drink-drive limit.

The 49-year-old grandfather admitted drink-driving, dangerous driving and neglect of child after the car was stopped in the middle of the day in January.

The man, who has name suppression, will be sentenced in July, the New Zealand Herald reports.

According to court documents, the seven-year-old was in the driver's seat and the man returned a breath-alcohol level of 1059 micrograms of alcohol per litre of breath. The legal limit is 400 micrograms.

Former Waitemata police serious crash unit head Stu Kearns labelled the case "ludicrous".

"There's no way a seven-year-old boy has got the skills to drive a motor vehicle," he said.