Motorists have been given a strong lead that tougher drink-driving laws are on the way.

Transport Minister Steven Joyce yesterday described existing legal alcohol limits for drivers as "ridiculous".

Speaking to a conference of traffic experts in Auckland, Mr Joyce said he could drink three-quarters of a bottle of wine in 90 minutes yet still have every chance of being under the legal alcohol limit for adult drivers.

"That's just ridiculous," he told the Local Authority Traffic Institute conference.

But he said heavy advertising when the existing adult limit of 80mg of alcohol per 100ml of blood was introduced in 1978 had made it difficult for him to gain popular acceptance for a further cut.

"A huge amount of advertising was done at the time which said it was just a couple of drinks and then one an hour."

The result was a popular misconception that reducing the limit to 50mg - one of 61 ideas suggested in a Ministry of Transport discussion paper - would restrict motorists to little more than one standard drink if they wanted to drive home.

"We have a problem here, because we have to tell them that no, it's a lot more than that," Mr Joyce said.

He said he wanted to hear a full range of community views on the various proposals before the Government committed itself to rule changes to slash the road toll by 2020.

Community support was essential for any change to succeed, the minister said, and there was no point "in us popping our heads up one day" and telling the public a new limit was in force.

"They're not going to have that," he said.

"So we have to get that point across much more aggressively and that's a challenge for me."

The first moves towards rule changes would be ready by about October next year, "so there is going to be a big discussion".

Mr Joyce told the Herald last night that his ultimate support for a lower alcohol limit could not be assumed, despite his concern at the amount drivers could drink under the present law.

He had based his wine calculation on advice in the ministry's "Safer Journeys" discussion paper that the existing limit allowed adult men to have six standard drinks within 90 minutes.

He described himself as a "social drinker" and could not recall having ever consumed that amount of alcohol within 90 minutes.

The discussion paper gives six drinks as the allowance for a man of average height and weight. For a woman, the limit is four drinks.

It says a limit of 50mg of alcohol, based on Australian guidelines, would allow an average man to have two drinks in the first hour and one an hour thereafter.

The discussion paper suggests a zero-alcohol tolerance for drivers younger than 20 and tightening the graduated licensing system.

It also proposes raising the minimum driving age to 16 or 17.

Public submissions on the paper close on October 2.

People on learner or restricted licences are already forbidden to drink any alcohol before driving.

University of Otago alcohol researcher Dr Jennie Connor said the legal limits were somewhat misleading.

She said anyone who had even one drink and then got behind the wheel of a car could have a crash.

"People who've had one drink are affected. Alcohol will affect everybody differently - what is safe for one person might not be safe for another person. So in that way, I think the limit is misleading.

"The more you drink, the worse your ability to drive is."

Dr Connor said the ideal solution was to separate the idea of drinking alcohol and driving completely, but she acknowledged that that could never happen.

* How much?

Beer: One 330ml can, 5 per cent alcohol equals 1.3 standard drinks.

Wine: 750ml bottle, 13 per cent alcohol equals 7.7 standard drinks.

RTD (ready to drink): 335ml bottle, 8 per cent alcohol equals 2 standard drinks.

Source: Alcohol Advisory Council

- ADDITIONAL REPORTING: Vaimoana Tapaleao