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.
Road safety campaigner Clive Matthew-Wilson, who also edits car review website, dogandlemon.com, was shocked to learn the practice was legal and called for a law change.
"It's completely crazy. I just can't believe our lawmakers are that stupid," he said. "It's like saying you're allowed to drive carrying a handgun as long as you're not shooting anyone.
"One tends to follow the other. If you're drinking while driving, then it's highly likely that it's going to cause serious road safety problems."
Assistant Commissioner Road Policing Dave Cliff highlighted Ministry of Transport figures that showed drinking and driving killed 1463 people and injured 24,789 others in road crashes between 2000 and 2011.
Anyone found over the legal limit while behind the wheel would face the "full pressure of our resources to remove them from the road". Vehicle owners may also find that their insurance does not cover them for alcohol consumption behind the wheel. Mr Cliff noted that police do not make the law.
A spokesman for the Transport Agency said: "Consuming any amount of alcohol impairs your ability to drive."
Nigel Hampton, QC, said motorists may not be breaking the law by drinking a beer while driving, but police could have "very good cause" to pull them over.
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