Teenage binge drinking has "dropped off the cliff" in the last 20 years but until now no-one really knew why.

A new paper, ‘What explains the decline in adolescent binge-drinking in New Zealand?' comes up with some answers but the reasons are complicated, surprising and sometimes contradict themselves.

Lead researcher Jude Ball, research fellow at University of Otago in Wellington calls it a ‘puzzle' and says some of the pieces "stubbornly don't fit".

She tells RNZ The Detail's Sharon Brettkelly about the one key factor behind the decline in binge drinking, and explains other influences, but stresses more work is needed to understand the trends.

Binge drinking in secondary school children started to decline around 2000, mirroring trends in other high income countries. Figures show 40 per cent of teenagers were binge drinking in 2001 and by 2012 it had nearly halved to 23 per cent.

Jude 's study, published in the International Journal of Drug Policy, is based on the ongoing Youth 2000 National Youth Health and Wellbeing Surveys collected in 2001, 2007 and 2012. The 2019 survey will be released this week.

Definitions of binge drinking vary but the Youth 2000 survey says it is five or more drinks in a session. The students were asked: 'In the past four weeks, how many times did you have five or more drinks in one session - within four hours.”

Ball's research focusses on adolescents under 16 and what's behind the major shift in risky behaviour among that group.

"Teenage pregnancy has declined, adolescent crime rates are down, dangerous driving. It's a really fascinating puzzle, particularly the alcohol aspect, because if anything our alcohol environment has liberalised in recent years," she says.

At the same time some ‘risk factors' associated with binge drinking had changed, such as teens in part time jobs.

"Having a part-time job, interestingly, is a risk factor for binge drinking because I guess if you've got more money then alcohol becomes more affordable. We knew that the proportion of secondary school students with a part-time job had fallen dramatically over this period."

Parental monitoring has increased; parents knowing where their children are and who they're with at all times.

"Parents do keep closer eye on their teenagers now than in the past."

The school environment has also changed in that time with students feeling more of a "belonging" than they had in the past and staying at school longer.

But Jude found something else played a much bigger role in the "phenomenal" drop in binge drinking among young teens.

The research also shows that what's going on in New Zealand doesn't always match what is happening in other countries.

Find out more about that in today's Detail - and the sting in the tail that highlights the urgent need for more research.

RNZ.