KEY POINTS:
Men would have to cut their drinking in half, and women by two-thirds, if they wanted to drink and drive under the new limits proposed by the police.
Guidelines in Australia, where all states have adopted the proposed limit of 50mg of alcohol for every 100ml of blood since the 1980s, say that men can drink only two "standard drinks", and women only one drink, in the first hour of drinking before going over the limit.
The current New Zealand limit of 80mg per 100ml allows men up to four drinks, and women up to three, in the first hour. Both limits allow up to one further drink an hour if you keep on drinking after the first hour.
The maths is approximate. The head of the New Zealand section of the Royal Australasian College of Physicians' addiction medicine chapter, Professor Ross McCormick, says the amount you can drink depends on your weight, how much your body has got used to drinking, and how much you have eaten.
"If you are heavier, you tend to be able to store more alcohol in your body," he said. A bigger person can drink more than a small one.
"The ability to detoxify also varies depending on how used to alcohol you are, so you can detoxify a little bit faster if you become a higher alcohol drinker.
"And if you had a big meal with four standard drinks, you'd have a lower blood alcohol content because it takes longer to absorb. The advice is that anybody who drinks up to that limit should have a meal with it to ensure that the blood alcohol content is lower than it would otherwise be."
A "standard drink" contains 10g of alcohol - roughly the amount of alcohol that the body can absorb out of the blood in an hour.
If you have four standard drinks in the first hour, or one every 15 minutes, that means you might have absorbed three-quarters of the first drink by the end of the hour so you will still have about 32.5g of alcohol in your blood.
In a "standard" 64kg person with 4.4 litres of blood in their body, that's 74mg for every 100ml of blood. A fifth drink would put you over the 80mg limit. If you have only two standard drinks in the first hour, or one every 30 minutes, your body might have absorbed only half of the first drink by the end of the hour so you would still have 15g of alcohol in your blood. For the same "standard" person, that's 34mg for every 100ml of blood and a third drink would put you over the proposed 50mg limit.
Professor McCormick said the College of Physicians supported a 50mg limit because that was the point at which most people's brains started to fuzz over.
"The number of drinks you can have with the current limit is actually quite high. If I took four in the first hour, I'd be feeling quite woolly in the head," he said. "If I take two, I'm actually quite clear."
The college says a driver with 50mg of alcohol in every 100ml of their blood is twice as likely to crash as one with no alcohol - but one with 80mg of alcohol is seven times more likely to crash.
"In any exponential risk curve the curve starts to take off, and by the time you get to 80mg per 100ml it's at the bottom of the part where the risk goes straight up," the professor said.
Sixteen of the 30 countries in the Organisation for Economic Cooperation and Development (OECD) now have drink-driving limits of 50mg per 100ml of blood. Only six others besides New Zealand (Britain, Ireland, Luxembourg, Canada, Mexico and the US) still have a limit of 80mg.
Four have limits of 20mg or 30mg, and three have zero tolerance of any alcohol in a driver's blood at all.
Apart from New Zealand, only the US has a separate limit for young people (20mg for people under 21, the US drinking age). But Australia, Canada, and Spain have lower limits (10mg-30mg) for new drivers (counted as "youth" in the table).
Even New Zealand's existing 30mg limit for drivers under 20 is considered so low that even a single standard drink would put most people over the limit.
Other OECD limits
Permissible mg of alcohol per 100 ml of blood
ADULTS
Britain 80
Ireland 80
Luxembourg 80
Mexico 80
New Zealand 80
USA 80
France 50
Spain 50
Australia 50
Japan 30
Norway 20
Czech Republic 0
Hungary 0
Slovak Republic 0
YOUTH
Britain 80
Ireland 80
Luxembourg 90
Mexico 80
New Zealand 30
USA 20
France 50
Spain 30
Australia 20
Japan 30
Norway 20
Czech Republic 0
Hungary 0
Slovak Republic 0
