An overwhelming majority of people support raising the legal purchase age for alcohol to 20 for bars and liquor shops, including supermarkets, according to the latest Herald-DigiPoll survey.

The buying age has been the subject of intense debate and will come into the spotlight soon when the Government pushes through its reforms to reduce alcohol-related harm.

The Government is proposing a split age - 18 for on-licence premises such as bars, 20 for off-licences such as liquor outlets - in the Alcohol Reform Bill, which is before the justice and electoral select committee.

When asked by the pollsters to choose between three options for the minimum age to buy alcohol, 58.6 per cent preferred 20, which was the age before a law change in 1999. Only 14.5 per cent wanted the status quo of 18, while 25.7 per cent wanted a split age.

A Law Commission report, which was the basis for the Government's bill, recommended a purchase age of 20, and said the change in 1999 had led to more drinking at a younger age and an increase in alcohol-related harm.

"There was a significant increase in hospital presentations of intoxicated people aged under 20 in the year following the law change" and in alcohol-related road crashes among 15- to 19-year-olds, it said.

In more than 2000 submissions to the commission, 68 per cent preferred a purchase age of 20, while 12 per cent backed a split age. A Massey University poll in 2006 found 75 per cent supported a purchase age of 20, while a Colmar Brunton poll last June found 74 per cent in support of 20.

The Government also proposes a new offence for parents who supply alcohol to under-18s in a private home without the consent of the young people's parents or guardians.

Almost 80 per cent supported this, but 18.5 per cent opposed it. Committee chairman Chester Borrows said officials were still working out how consent should be obtained.

A conscience vote - one where MPs do not have to follow a party line - is expected when the bill reaches the committee stage in the House, as MPs will likely table amendments in support of the status quo or an age of 20.

The Herald-DigiPoll survey was conducted in the week to Tuesday and had 750 respondents. The margin of error is 3.6 per cent.