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Prime Minister John Key says he would be "a bit messy" and quite possibly asleep after drinking the amount of alcohol a Weekend Herald test suggested could leave a driver legally entitled to get behind the wheel.

After the Government decided to leave the current breath alcohol limit for drivers at 400 micrograms of alcohol per litre of breath rather than the 250 micrograms recommended by the Ministry of Transport, the Weekend Herald conducted a test to see how much drinking would take a driver over the current legal limit.

Weekend Herald photographer Richie Robinson, weighing 85kg, drank nine bottles of lager amounting to 11.7 standard drinks in just under four hours before reaching a reading of 400mcg of alcohol per litre of breath.

Yesterday, Mr Key said he would not be prepared to take the same test himself, "because I need to run the country the next day and I'd be a bit messy after nine beers".

While he couldn't be sure, Mr Key believed he would be over the limit after consuming that much.

"It would depend on a number of factors. Certainly I tend to get not enough sleep these days so I would have thought I might fall asleep before I get to the ninth beer. "

He said it was important to remember the blood alcohol limit was not a target that people should be aiming at.

"I think most people set a limit for themselves that they feel they are in control of a motor vehicle and provided that's under the legal limit then it's for them to assess what they think that right level is."

While he rarely drove a car these days, when he did, "I would never have more than two beers or one glass of wine".

Mr Key said it would be "very dangerous for people to pick up a copy of the Weekend Herald and believe that they can have nine beers over four hours and not necessarily be over the limit because in some cases they will be".

This was because reaching the limit would depend on variables including how tired the driver was, their weight, how much they ate, as well as alcohol consumption,

The fact that 100 people were being caught or convicted for drink drive offences each day suggested they were either drinking an enormous amount or the effect of drinking varied considerably on the individual, he said.

The Weekend Herald's test showed that had the limit been set at the Ministry of Transport's recommendation of 250mcg limit, Robinson would still have been able to drink almost six bottles of beer.

Reporter Beck Vass, at 59kg, remained slightly within the existing limit after drinking five glasses of wine in just under two hours.

She would have passed 250mcg on her third glass.

Transport Minister Steven Joyce last week said the Government would review its decision on limits after two years of local research.