Can I drive the morning after drinking? A New Zealand guide

You’ve had a big night, you’ve slept, you feel more or less fine. Are you safe to drive?

Maybe. But maybe not — and “maybe” is how a lot of people end up with a drink driving charge on the way to work.

Sleep doesn’t sober you up

Your liver clears alcohol at a fairly steady rate: roughly one standard drink per hour for most people. Sleeping doesn’t speed that up. Neither does coffee, a shower, a greasy breakfast, a run, or drinking water. They might make you feel better, but the alcohol in your blood is leaving at the same pace regardless.

So if you finish drinking at 1am with a lot of alcohol still in your system, it doesn’t matter that you went to bed — your body has only had the hours since then to work through it.

Counting standard drinks

In New Zealand a standard drink is 10 grams of pure alcohol. The catch is that most drinks are more than one standard drink:

330 mL bottle of 5% beer: about 1.3 standard drinks
750 mL bottle of 12.5% wine: about 7.4 standard drinks
A “glass” of wine poured at home: often 1.5–2 standard drinks
A pre-mixed 7% RTD can: about 1.4–1.8 standard drinks, depending on the size

The number of standard drinks is printed on every New Zealand alcohol label. It’s worth reading — most people are surprised at how fast it adds up.

A rough example

Say you drink a bottle and a half of wine over an evening. That’s around 11 standard drinks. Even allowing for the alcohol your body cleared while you were still drinking, there could be several hours’ worth left when you go to bed — and for a lot of people, that runs past the time they’d normally leave for work.

Everyone is different, which is exactly why rules of thumb don’t work well. Your weight, sex, whether you ate, how quickly you drank and your own metabolism all change the answer. Your estimate can easily be out by a few hours.

Under 20? Morning-after matters even more

If you’re under 20, the limit is zero. Even a small amount of alcohol left over from the night before is an offence. See NZ drink driving limits explained for the full rules.

How to know for sure

The only reliable way to know whether there’s still alcohol in your system the next morning is to test.

Wait at least 15–20 minutes after brushing your teeth, using mouthwash, eating or drinking anything. Leftover alcohol in your mouth (including from some mouthwashes) can give a falsely high reading.
Test with a fuel cell breathalyser. Cheaper semiconductor devices can be thrown off by other substances on your breath and drift out of accuracy — see fuel cell vs semiconductor breathalysers.
Read the result honestly. If you’re 20 or over and you’re anywhere near 250 mcg, or if you’re under 20 and the reading isn’t zero, don’t drive. Test again later.

Make a plan the night before

The easiest morning-after is the one you plan for. If you know you’ll be drinking, sort out the next morning in advance: a later start, a lift, a bus, or a taxi. Then test before you get behind the wheel rather than after you’ve already decided you’re fine.

A personal breathalyser like the AF-35C costs less than a single drink driving fine, and it gives you an answer in under 30 seconds.

This article is general information, not medical or legal advice. Breathalyser readings are a guide — if in doubt, don’t drive.