A couple who reported a man driving with an open bottle of beer in Nelson were dumbfounded to be told by police he wasn't breaking the law.
The legal loophole was confirmed by the area's road policing manager, Inspector Jenni Richardson, who said people aged 20 or more could drink while driving providing they did not exceed the legal limit.
However, drivers under 20 years must adhere to a zero alcohol limit, she said.
"While it is not illegal, police do not encourage people to drink alcohol while driving and they would certainly stop them and check their alcohol consumption," said Ms Richardson.
She urged road users concerned about the behaviour of other drivers to note their registration details and call police immediately.